362 



NORTH AMERICAN INDEX FOSSILS. 



The opening of the valve is brought about by an external elastic 

 ligament stretched across the hinge line from valve to valve, or by 

 an internal cartilage or resilium. The ligament may extend on 

 both sides of the beak {amphidetic — Glycimeris) or be only behind 

 the beak [opisthodetic — Venus). In the amphidetic type, a flattened 

 area is often developed under the beaks. This may separate into 

 a posterior escutcheon carrying the ligament as in the Teleodonta, 

 and into an anterior lunule. The ligament may consist of a single 

 elastic strand stretched across from beak to beak {alivincular — 

 Lima), or there may be many such (inultivincular — Perna, Area, 

 etc.), or it may consist of a split cylinder in the form of a C spring 

 (j)arivincular — Tellina, Veitus). The internal cartilage or resilium 

 may be lodged in cartilage pits or resilifers (also called chondro- 

 phore) (Mactrd) or supported by variously formed calcareous 

 pieces [lithodesmd). The closing of the valves is produced by the 

 contraction of closing or adductor muscles. 



The scars marking the attach- 

 ment of the adductor or closing 

 muscle, or muscles, vary greatly, 

 and are frequently preserved in the 

 fossil forms. When two are pres- 

 ent they are designated respectively 

 as the anterior and posterior adduc- 

 tor scars. The line of attachment 

 of the fleshy mantle which builds 

 the shells, i. e., the pallial line, is 

 often visible. Near the posterior 

 end it frequently makes a reentrant 

 curve — the pallial sinus — indicat- 

 ing that the animal had a retractile 

 siphon. The various parts described 

 are indicated in Fig. 476. 

 The principal soft parts of the animal comprise : the mantle, 

 consisting of two fleshy folds, one lining each valve, and building 

 it ; the abdomen, with the anteriorly placed mouth, and the antero- 

 ventral foot ; the gills or branchial, which consist of complicated 

 lamellae hanging on either side of the abdomen in the mantle 

 cavity; and the siphons — present only in certain forms — pos- 

 teriorly placed, often capable of great extension, and serving, the 



Fig. 476. Diagrammatic view of 

 left valve of Cytherea ; (a. a.) ante- 

 rior adductor scar; {p. a.) posterior 

 adductor scar; (/./•) pallial line; 

 •(/••*•) pallial sinus; (/.) ligament; 

 (t. ) teeth; (urn.) umbo or beak. 

 (After Lang, adapted.) 



