MOLLUSCA — PELECYPODS 



223 



Derivation of name. — Greek pelekus, an ax, -\- pous {pod), 

 foot. The foot of many pelecypods is shaped like an ancient 

 battle ax. 



Nucula (Fig. 92). Ordovician to present. 



Valves small, oval or triangular, with a pearly interior. 

 Hinge line taxodont, i.e. bearing a series of small, nearly equal, 

 transverse teeth. Beak exceptional in facing the posterior 

 end of the shell, i.e. 

 the beak faces away 

 from the mouth, 

 which is near the an- 

 terior edge of the 

 shell. Mantle lobes 

 free, without siphons, 

 hence pallial line 

 simple. Two adduc- 

 tor muscles present. 

 Name from Latin 

 nucula, a little nut. 



Especially abun- 

 dant in the Hamilton, Pennsylvanian and Cretaceous of North 

 America, but widely represented by both fossil and recent forms 

 in Europe and Asia as well. 



1. How do the beaks of Nucula differ from those of most other 

 pelecypods ? How is this known ? 



2. Sketch an interior view of one valve, indicating anterior 

 and posterior ends, beak, hinge line, teeth, muscle scars, pallial 

 line. 



3. What is the function of the teeth ? 



4. Mention three characters which determine this to be a 

 pelecypod shell rather than a brachiopod. 



5. What is the probable origin of teeth in pelecypods ? 



Inoceramus (Fig. 93). Jurassic to Cretaceous. 



Valves unequal. Hinge line long, straight, without teeth, 



but with numerous small, transverse ligament pits. Surface 



Fig. 92. — Nucula proxima Say, from the Miocene of 

 Maryland. Natural size. A, exterior of left valve. 

 B, interior of right valve showing the two muscle 

 scars and the numerous teeth in a row (taxodont 

 dentition), a.a., anterior adductor muscle scar; 

 p.a., posterior adductor muscle scar. (From 

 Glenn.) 



