278 MOLLUSCA. 



M. de Blainville also separates the 



CoEAliLiopHAGA, Blainv. 



Where the shell is thin, and the lateral plate considerably effaced, which 

 may cause their approximation to Venus. 



One of them is known which excavates coralline masses to form its habi- 

 tation. The 



Venericardia, Latn. 

 Only differs from the Carditx in the circumstance that the posterior plate 

 of their hinge is shorter and more transverse, which caused their approxi- 

 mation to Venus; their form is almost round. Judging- from the impressions 

 of its muscles on tliem, their animal must resemble that of the Carditse and 

 Unios. 



Both of them approach the Cardia in their general form and the direction 

 of their ribs. I suspect that this is also the place for the 



Crassatella, Lam. 



"Which has sometimes been approximated to Mactra, and at others to 

 Venus; the hinge has two slightly marked lateral teeth, and two very strong 

 middle ones, behind which, extending to both sides, is a triangular J^cavity 

 for an internal ligament. The valves become very thick by age, and the 

 impression made by the margin of the mantle, leads to the belief that there 

 are no protractile tubes. 



FAMILY III. 



CHAMACEA. 



The mantle closed and perforated by three holes, through one of 

 which passes the foot; the second furnishes an entrance and exit to 

 the water requisite for respiration: the two latter are not prolonged 

 into tubes as in the subsequent family. It only comprises the genus 



Chama, Lin., 



Where the hinge is very analogous to that of a Unio, that is to say, the left 

 valve near the summit is provided with a tooth, and further back with a sa- 

 lient plate, which are received into corresponding fossse of the right valve. 

 This genus has necessarily been divided. In the 



Tkidacna, Brug., 



The shell is greatly elongated transversely, andequivalve; the superior angle 

 which answers to the head and summit, very obtuse. 



