63 



CRENATV LA.—Lamarck. 



Gen. Char. A subequalvalved, bivalve, flat- 

 ish, lamellose, rather irregular; hinge 

 lateral, linear, internally marginal, crenu- 

 lated ; crenulae arranged in a series, cal- 

 lous, spoonformed, receiving a disjointed 

 Ligament, 



X HE recent species of this Genus are flat shells, and 

 composed of Lamellae, of a distinctly fibrous structure, 

 which do not project beyond the surface, but mark upon 

 it numerous, sharp lines much resembling those upon 

 luoceramus ; the hinge contains the characters in which 

 it differs, it consists of a number of spoonlike pro- 

 cesses, some of which, near the beaks, are placed close 

 together, while others are distant, they receive de- 

 tached portions of the ligament, and are not supported 

 upon a distinct callus that, after bein^ bent, extends be- 

 yond the beaks as in Inoceramus ; those processes that 

 are united together near the beaks, have still a furrow 

 between the edges of their upper hollow sides, to which 

 no ligament is attached ; this sulcus does not exist in 

 Inoceramus, where one continued credulated grove in 

 each valve appears constructed to receive an undivided 

 ligament ; in Crenatula there is no great disproportion 

 of thickness between the shell, about the beaks and mar- 

 gin. Perna is distinguished from both the above men- 

 tioned Genera, by having a sinus for the passage of a 

 byssus. 



For other particulars my Brother's work upon Genera 

 may be consulted ; the above are sufficient to discrimi- 

 nate fossils by. 



