ACEPHALA TESTACEA. 265 



CYPRICARDIA, Lamarck. 



Carditse, in which the tooth under the summit is divided into two or three. 

 Their form is oblong, and then- sides unequal 

 M. de Blainville also separates the 



CORALLIOPHAGA, De Blainville, 



Where the sheh 1 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 habita- 

 tion. The 



VENERICARDIA, Lamarck, 



Only differs from the Carditae 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 them, their animal must resemble that of the Carditse and 

 Unios. 



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

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



CRASSATELLA, Lamarck, 



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 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 requi- 

 site for respiration: the two latter are not prolonged into tubes as in the 

 subsequent family. It only comprises the genus 



CHAMA, Linnaeus, 



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 salient 

 plate, which are received into corresponding fossae of the right valve. This 

 genus has necessarily been divided. In the 



TRIDACNA, Bruguieres, 



The shell is greatly elongated transversely, and equivalve; the superior angle, 

 which answers to the head and summit, very obtuse. 



