GENERA OF SHELLS. 47 



Sect. II. — Ligament not marginal, contracted 



INTO A SMALL SPACE UNDER THE BEAKS ', ALWAYS 

 VISIBLE, AND NOT FORMING A TENDINOUS CORD 

 UNDER THE SHELL. 



These shells are generally auriculated at the base or 

 extremity of the cardinal margin. They are all ine* 

 quivalve, for though in many the valves are of the 

 same size, one of them is always more convex tlian the 

 other. 



PECTENIDES. 



Ligament interior or semi-interior ; shell generally regular, 

 compact, not foliated. 



1^- Pedum. Shell inequivalve, somewhat eared, 

 inferior valve gaping ; beaks unequal, distant. 

 Hinge toothless ; ligament partly exterior, in- 

 serted in an elongated, canal-shaped fossa, 

 under the beaks ; inferior valve emarginate 

 near its posterior base. 



The pedum is a free, regular, inequivalve shell ; 

 and the singular notch of the lower valve shows that 

 the animal has tlje i)ower of attaching it by a byssus. 



P. spondyloideum. 



Lima. Shell longitudinal, subequivalve, 

 eared, slightly gaping on one side between 

 the valves ; beaks distant, internal face in- 

 clined outwards. Hinge toothless ; cardinal 

 fossa partly exterior, receiving the ligament. 



The lima has no notch on the lower valve ; the little 

 ears, though small, are distinct. It differs from the 



