256 CARDITID.E. 



Family X. CARDITIDjE, Gray. 



Body oblong or roundish -oval : mantle thin, open in front, 

 and folded behind into a short excretal tube : foot tongue- 

 shaped and extensile, byssiferous. 



Shell shaped like the animal, equivalve, inequilateral : 

 ligament external: hinge strong, furnished with one or two 

 cardinal teeth, which are set more or less obliquely, and with 

 a laminar posterior lateral tooth in each valve : pallial scar in 

 nearly every case entire: muscular scars large, round, and 

 deep. 



I give this family provisionally as British, not know- 

 ing where else to place the abnormal genus Cyamium, 

 as well as a species of Cypricardia, which at present I 

 am not sure is indigenous. The present members of this 

 family are dispersed over the ocean, but chiefly confined 

 to the southern zone : their ancestors nourished in the 

 northern hemisphere, and multiplied exceedingly during 

 the tertiary epoch. 



With respect to the shape of the pallial scar, being 

 the impression produced by the adhesion of the mantle 

 to the inside of the shell near the front margin, I think 

 an undue importance has been attached to this cha- 

 racter. When it is sinuous or indented, it certainly 

 shows that the animal has two distinct tubes springing 

 from a common sheath, at the posterior side ; but when 

 it is simple, or even flexuous, we cannot learn from it 

 whether there are two such tubes, or only one, nor 

 what their length may be, nor even whether there is 

 any tube at all. Cardium and Venus have each a double 

 tube on the posterior side ; but the pallial scar in the 

 former is simple or entire, and in the latter sinuous. 



