# Phylum Mollusca ## expand\_more ### Class Bivalvia expand\_more #### MORPHOLOGY Images taken from Keen, 1969 and Cox, 1969 The images below display basic features of bivalve morphology, including internal and external characteristics. ![Internal Morphology (1a right valve interior view, 1b left valve exterior view)](/sites/g/files/omnuum8411/files/styles/hwp_1_1__360x360_scale/public/trenton/files/bivalvemorpho.jpg?itok=E0GgLaI0) Internal Morphology (1a right valve interior view, 1b left valve exterior view) ![External Morphology (1a right valve interior view, 1b dorsal view, 1c left valve external view)](/sites/g/files/omnuum8411/files/styles/hwp_1_1__360x360_scale/public/trenton/files/bivalvemorpho2.jpg?itok=ZTJUxaPr) External Morphology (1a right valve interior view, 1b dorsal view, 1c left valve external view)#### **TERMINOLOGY** *Definitions of terms taken from Cox, 1969* **Adductor muscle** Muscle, commonly one of 2, connecting 2 valves of shell, tending to draw them together. **Anisomyarian musculature** With one adductor muscle (anterior) much reduced or absent. **Auricle** Earlike extension of dorsal region of shell, commonly separated from body of shell by notch or sinus. **Auriculate** With auricles. **Beak** Noselike angle, located along or above hinge margin, marking point where growth of shell started. **Byssal notch** Indentation below anterior auricle of right valve in many Pectinacea (scallops) for passage of byssus (a bundle of hairlike strands used for attachment) or protrusion of foot (a muscular structure for burrowing and locomotion). **Byssally attached** Temporary attachment made by bivalves to extraneous objects by means of a bundle of hairlike strands. **Carina** Prominent keel-like ridge. **Cardinal area** Flat or slightly concave, commonly triangular surface extending between beak and hinge margin in many bivalves, and partly or wholly occupied by ligament. **Cardinal teeth** Hinge teeth situated close to the beak. **Concentric** With direction coinciding with that of growth lines. (By no means concentric in literal and geometrical sense of term). **Costellae** Rather narrow linear elevations of the surface of shell. **Duplivincular ligament** Type of ligament with lamellar component repeated as series of bands, each with its 2 edges inserted in narrow grooves in cardinal areas of respective valves. **Equilateral** With parts of shell anterior and posterior to beaks equal in length or almost so. **Equivalved** With 2 valves of same shape and size. **Gape** Localized opening remaining between margins of shell when valves are drawn together by adductor muscles. **Growth line** Line on the surface of shell, one of usually irregularly arranged series, marking position of margin at some stage in growth. **Hinge** Collective term for structures of dorsal region which function during opening and closing of valves. **Hinge tooth** Shelly structure (usually one of a series) adjacent to dorsal margin and received in socket in opposite valve; hinge teeth serve to hold valves in position when closed. **Inequilateral** With parts of shell anterior and posterior to beaks differing appreciably in length. **Ligament** Horny elastic structure or structures joining 2 valves of shell dorsally and acting as spring causing them to open when adductor muscles relax. **Lunule** Depression, commonly cordate in shape, present anterior to beaks in many bivalves. **Mantle** Integment that surrounds vital organs of mollusk and secretes shell. **Modioliform** Shaped like shell of Modiolus; differing from mytiliform in that beaks are not quite terminal and anteroventral region forms slight bulge. **Muscle scar** Impression on anterior of shell marking former place of attachment of a muscle. **Opisthocline** Sloping (from lower end) in posterior direction (term applied to hinge teeth and, in some genera, to body of shell). **Opisthogyrate** Curved so that beaks point in posterior direction. **Pallial line** Line or narrow band on interior of valve close to margin, marking the line of attachment of marginal muscles of the mantle (integment that surrounds vital organs). **Pallial sinus** Line or narrow band on interior of valve close to margin, marking line of attachment of marginal muscle of mantle. **Posterior lateral tooth** Lateral tooth situated posteriorly to beaks and (in heterodonts which have distinctly differentiated cardinal and lateral teeth) posteriorly to ligament. **Prosogyrate** Curved so that beaks point in anterior direction. **Radial** Direction of growth outward from beak at any point on surface of shell, commonly indicated by direction of costa (a broad prominent elevation) or other element of ornament. **Resilifer** Recess or process for attachment of internal ligament. **Siphonate** Having a siphon (a tubelike extension of mantle for passage of inhalant or exhalent current). **Socket** Recess for reception of hinge tooth of opposite valve. **Solemyiform** Recess for reception of hinge tooth of opposite valve. **Sulcus** Lateral depression of the surface of shell. **Truncate** With curvature of outline interrupted by straight cut. **Valve** One of the calcareous structures (2 in most bivalves) of which shell consists. ### Class Cephalopoda (Subclass Nautiloidea) expand\_more #### MORPHOLOGY Images taken from Teichert, 1964 The labeled images below display some morphological attributes of cephalopods, specifically of the nautiloids. ![Diagram of an Orthoconic Nautiloid. (The septa are spaced widely in the anterior of the phragmacone to show thier shape.)](/sites/g/files/omnuum8411/files/styles/hwp_1_1__360x360_scale/public/trenton/files/nautiloiddiagram1.jpg?itok=D6gyM1UG) Diagram of an Orthoconic Nautiloid. (The septa are spaced widely in the anterior of the phragmacone to show thier shape.) ![Various nautiloid shell shapes](/sites/g/files/omnuum8411/files/styles/hwp_1_1__360x360_scale/public/trenton/files/nautiloiddiagram4.jpg?itok=-uKbpPlp) Various nautiloid shell shapes ![Sketch showing the possible orientations of cephalopod shells. ( A ](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/nautiloiddiagram3.jpg?itok=B-RgJ1Jj) [*Sketch showing the possible orientations of cephalopod shells ( A "conventional", B "hypothetical")*](https://static.hwpi.harvard.edu/files/styles/os_files_xlarge/public/trenton/files/nautiloiddiagram3.jpg?m=1525726168&itok=xlZkqhqt)#### TERMINOLOGY *Definitions of terms taken from Teichert, 1964* **Actinosiphonate** Descriptive of endosiphuncular deposits consisting of radially arranged longitudinal lamellae. **Annulation** Ringlike expansion of conch, either transverse or slightly oblique to longitudinal axis of conch. **Annulosiphonate deposits** Descriptive of endosiphuncular deposits having shape of rings formed at septal foramen and attached to inside of septal neck. **Aperture** Opening of shell through which head-foot protrudes. **Body chamber** Large undivided, anterior space in conch at open aperture, occupied by animal’s body. **Brevicone** Short rapidly expanding conch. **Camera (pl., camerae)** Space enclosed between two adjacent septa but not including siphuncle. **Cameral deposits** Calcareous deposits secreted against original walls of camerae during life of animal. **Ciruculus** Cameral deposit on concave surface of cyrtochoanitic septal neck. **Conch** All hard calcareous parts secreted by nautiloid animal including external shell, septa, and siphuncle, but not cameral deposits and any structures within the siphuncle (syn., test). **Conchal furrow** Shallow groove on inside of conch wall, located mid-ventrally. **Connecting rings** Partly calcareous, partly conchiolinous, tubular membrane, that connects septal neck of ellipochoanitic (having relatively short retrochoanitic septal necks which do not reach as far as preceding septum) conchs with septum immediately behind it; vestiges of it are also found in holochoanitic forms. **Cyrtochoanitic** Descriptive of comparatively short, retrochoanitic septal necks which are curved so as to be concave outward. **Cyrtocone** Curved conch that completes less than one whorl. **Dorsomyarian** Descriptive term applied to nautiloids in which retractor muscles of head-foot are attached to shell along interior areas of body chamber adjacent to, or coincident with, its dorsal mid-line. **Dorsum** Side of animal or conch opposite venter. Among fossil nautiloids, if position of hyponomic sinus (and therefore venter) cannot be established, the term is loosely applied (1) in coiled forms to the concave side of a whorl, (2) in straight or curved conchs with eccentric siphuncle to the side farthest removed from the siphuncle. **Endocone** One of series of calcareous cones formed in the posterior or adapical portion of siphuncle mainly of endocerid and discosorid conchs. **Endosiphuncle** Space within ectosiphuncle (walls of the siphuncle) including all organic tissues and calcareous structures. **Endosiphuncular tube** (1) Circular tube formed by adapical thickening of lateral walls of endosiphocoleon (flattened tube or balde surrounding endosiphuncluar tube) in some Encocerida; (2) simple tubes connecting apices of endocones where no differentiation coleon and tube is possible. **Episeptal deposits** Cameral deposits on concave (or adapertural) side of septum. **Exogastric** Descriptive of conchs curved or coiled so that venter is on or near outer convex side. **Gibbous** A prominent swelling to one side. **Growth lines** Markings on surface of conch that denote periodic increases in size and hence former positions of aperture. **Gyroconic** Loosley coiled conch in successive whorls are not in contact with each other. **Holochoanitic** Descriptive of retrochoanitic septal necks that extend backward through length of one camera. **Hyposeptal deposits** Cameral deposits on convex (adapical) side of septum. **Hyponomic sinus** Large concave sinus in middle of aperture, marking location of hyponome, invariably ventral. **Lirae** Parallel fine ridges or raised lines on surface of conch, transverse or longitudinal, separated by striae. \[Restricted to features not easily discernible to the naked eye.\] **Mural deposit** Cameral deposits along mural parts of septa (the parts of the septum attached to wall of conch). **Nummuloidal** Having siphuncular segments that are strongly inflated between septa, mostly in Actinocerida. **Orthocone** Straight conch. **Othochoanitic** Descriptive of retrochoanitic septal necks which are straight, cylindrical, and extend only a short way to preceding septum. **Phragmocone** Chambered portion of conch. **Retrochoanitic** Descriptive of septal necks which are directed backward. **Septal foramen** Opening in septum allowing passage of siphuncular cord. **Septal neck** Portion of septum which is bent adapically (or backward) around septal foramen. **Septum** Partitions dividing phragmocone into camerae and attached to inside of wall of conch. **Sinus** Any part of transverse feature (apertural margin, ribs, growth-lines) concave toward aperture. **Siphuncle** Long slender or thick tube extending through all camerae to apex and consisting of soft and shelly parts, including septal necks, connecting rings, calcareous deposits, and siphuncular cord (adj., siphuncular). **Siphuncular** See siphuncle. **Striae** Parallel, small to minute grooves or channels on surface of conch, either transverse or longitudinal, separated by lirae. \[Restricted to features not easily discernible to the naked eye.\] **Suborthochoanitic** Descriptive of retrochoanitic septal necks that are short and straight, with slightly outwardly inclined tips but with no measurable brim. **Suture** Line of junction of free part of septum and inner side of phragmocone wall. **Test** See conch. **Torticone** Conch coiled in helicoidal spire, as in gastropods. **Venter** Underside of organism and of conch, distinguished generally by hyponomic sinus and often by conchal furrow. **Whorl** Complete turn of coiled conch (syn., volution). ### Class Gastropoda expand\_more #### MORPHOLOGY Image from Peel in Boardman, Cheetham, and Rowell, 1987 Labeled on the image below are many of the important morphologic traits of a gastropod shell. Note: Some of the features of these shells are not present on the Ordovician gastropods represented in the Trenton Group. ![Gastropod hard part anatomy](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/gastrapoddiagram.jpg?itok=JCis4cSR) [*Gastropod hard part anatomy*](https://static.hwpi.harvard.edu/files/styles/os_files_xxlarge/public/trenton/files/gastrapoddiagram.jpg?m=1525740358&itok=jwXV4Ny6)#### **TERMINOLOGY** *Definitions of terms from Cox, 1960* **Abapical** Away from shell apex toward base along axis or slightly oblique to it. **Adapical** Toward shell apex along axis or slightly oblique to it. **Aperture** Opening at last formed margin of shell, providing outlet for the head-fott mass. **Base** In conispiral shells ( with spire projecting as cone), part of surface lying on abapical side of periphery of last whorl or of a carina or angulation that forms an obvious lower boundary on the side of whorl. **Cancellate** Having ornament of intersecting spiral and transverse threads or cords. **Carina** Prominent spiral ridge or keel. **Columella** Solid or hollow pillar surrounding axis of a coiled shell, formed by adaxial walls of whorls. **Cord** Round-topped moderately coarse spiral or transverse linear elevation on shell surface. **Dextral** Right handed; term originally applied to any shell with aperture on observer's right when shell apex is directed upward, or with apparent clockwise coiling when viewed from above apex, but in fact definition depends on features of soft anatomy. A dextral gastropod has genitalia on the right side of the head-foot mass or pallial cavity and the shell of such an animal commonly has the aperature on the right when viewed with the apex uppermost. **Disjunct** Condition of whorls when out of contact. **Dorsum** Dorsal side; side opposite the venter. **Growth lines** Collabrally disposed surface markings of shell, generally not prominent as to relief, that denote former positions of outer lip. **Hyperstrophic** Dextral anatomically, with genitalia on the right, but shell appears as if the anatomy would be on the left side. **Inductura** Smooth shelly layer secreted by general surface of mantle, commonly extending from inner side of aperture over parietal region, columellar lip, and (in some genera) part or all of shell exterior. **Inner lip** Adaxial margin of aperture extending from foot of columella to suture and consisting of columellar and parietal lips. **Involute** With the last whorl enveloping earlier ones so that height (or width in shells like bellerophonts) of aperture corresponds to that of shell; early whorls more or less visible in umbilici. **Lamellae** Thin plates. **Lamellose** Having lamellae. **Lanceolate** Shaped like a lance-head (i.e., sharply pointed at one end), broader at the other. **Lunulae** Crescentic linear ridge on selenizone, concave toward aperture. **Nucleus** Earliest-formed part of shell. **Orthostrophic** Coiled in normal manner, not hyperstrophic. **Outer lip** Abaxial margin of aperture extending from suture to foot of columella. **Parietal region** Basal surface of helicocone just within and just without aperture; the redundant expression *parietal wall* should not be used, for *parietal* signifies pertaining to wall. **Peristome** Margin of aperture. **Phaneromphalous** With completely open umbilicus; may be wide, narrow, or very minute. **Ramp** Abapically inclined flattened band on shell surface, which in some shells forms the adapical part of whorls, limited abaxially by ridge or angulation. **Revolving** Passing spirally around whorls. **Selenizone** Spiral band of crescentic growth lines or threads (lunulae) generated by a narrow notch or slit and characteristic of dibranchiate gastropods. **Septum** Transverse plate secreted within early-formed whorls of some shells for closing them off. **Shoulder** Angulation of whorl forming abaxial edge of sutural ramp or shelf. **Sinus** Curved re-entrant of apertural margin or of growth lines. **Slit** Parallel-sided re-entrant of outer lip ranging from shallow incision to deep fissure as half a whorl in extent. **Spire** Adapical visible part of all whorls except last. **Sutural shelf** Horizontal flattened band, which in some shells adjoins adapical suture of whorls. **Suture** Continuous line on shell surface where whorls adjoin. **Thread** Fine linear surface elevation. **Trochiform** With flat-sided conical, not highly acute spire and rather flat base, like shell of Trochus. **Turbiniform** With broadly conical spire and convex base, as in shell of Turbo . **Umbilicus** Cavity or depression formed around shell axis between faces of adaxial walls of whorls where these do not coalesce to form a solid columella; in conispiral shells (except hyperstrophic ones) its opening is at base of shell but involute shells may have two umbilici. **Venter** Ventral side; the underside of the animal. **Ventricose** Inflated or swollen, especially to one side. **Volution** Any complete coil of helicocone (a distally expanding coiled tube that forms most gastropod shells). **Whorl** (1) Any complete coil of helicocone (a distally expanding coiled tube that forms most gastropod shells); (2) exposed surface of any complete coil of helicocone. ### Class Monoplacophora expand\_more #### MORPHOLOGY Image modified from Knight and Yochelson, 1960 ![A sketch of the Recent monoplacophoran Neopilina showing (A) the ventral side with labeled body parts and (B) the dorsal side showing details of the shell.](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/monoplacophorandiagram.jpg?itok=mrE4Q5Ea) [*A sketch of the recent monoplacophoran Neopilina showing (A) the ventral side with labeled body parts and (B) the dorsal side showing details of the shell.*](https://static.hwpi.harvard.edu/files/styles/os_files_xxlarge/public/trenton/files/monoplacophorandiagram.jpg?m=1525740981&itok=gH9rghZp)