MOLLUSCA. 1 85 



. Cavity of the shell interrupted by a testaceous plate. 

 This division consists of five genera, each of which may 

 be regarded as the type of a family, although, for the pre- 

 sent, they are all included in one. In the Crepidula, the 

 gills form a transverse ridge on the roof of the cavity, con- 

 sisting of filaments extending beyond the margin. The eyes 

 are at the base of the tentacula. There is only one fluvi- 

 atile genus, termed Navicella. 



B. Cavity of the shell entire. 



In the Capulus, the shell is entire, the foot is complicated 

 on its anterior margin. The shell adheres to the animal by 

 a circular muscle, leaving an opening in front, for the issue 

 of the head and entrance to the branchial cavity. The gills 

 form a single ridge across the roof. The mouth is in the 

 form of an extended proboscis, with a deep groove above. 

 The tentacula, which are two in number, have the eyes at 

 the external base. The anus is on the right side of the 

 branchial cavity. In the Carinaria, the foot appears to be 

 compressed, and formed for swimming. The head is co- 

 vered with a group of tubercles. The mouth is furnished 

 with a proboscis. Near the middle of the body the shell is 

 attached. The surface of the body above is closely covered 

 with small tubercles. It is probable that the species here 

 alluded to is the same with the Pterotracheq, coronata of 

 Forskal. 



Fissurellada. Shell with a slit or perforation. In the 

 Fissurella, the apex of the shell is perforated, and united to 

 the cloak by a circular muscle open in front. The cloak 

 forms a duplicature in front for the branchial cavity, which 

 extends to the perforated apex of the shell. The gills con- 

 sist of two ridges ; at the dorsal extremity of which is the 

 anus. It is probable that the excrements are ejected at the 



