238 



THE SEA SHORE 



Another rather common species (C. Icevis) is distinguished by the 

 glossy appearance of the dorsal portion of the shell. 



It will have been observed that the chitons differ from the 

 majority of gasteropoda in that their shells and bodies are both 

 bilaterally symmetrical, and the same is true of the next family 

 Dentaliadce, which derive their name from the tooth-like form of 

 their conical shells. They are popularly known as the Tooth Shells, 

 and although they generally live beyond low-water level, they may 

 sometimes be seen alive on the beach, and the empty shells are often 

 washed up by the waves. 



The shells (fig. 165) are curved, and open at both ends, the 

 narrower extremity being the posterior. The mouth is circular, and 

 the outer surface is quite smooth or grooved. 



In these animals, too, the head is imperfectly formed, without 

 eyes or tentacles. The foot is conical and pointed, with two 



FIG. 164. CHITON SHELLS 



FIG. 165. SHELLS OF Dentalium 



symmetrical side lobes; and the gills, also two in number, are 

 symmetrically disposed. The margin of the mouth is fringed, and 

 the animal is attached to the shell near the posterior end. 



The DentaliadcB are carnivorous, subsisting on minute molluscs, 

 foraminifera, &c., and generally live on sandy or muddy bottoms, in 

 which they sometimes bury themselves. 



Our next family includes the familiar Limpets, and is. designated 

 Patellidce on account of the resemblance of the conical shell to 

 a little dish. In these the apex of the cone is not central, but 

 situated more or less towards the anterior ; and the muscular 

 impression within is shaped like a horseshoe, with its open end 

 turned to the front. 



Unlike the members of the preceding families, the limpets have 

 a well-formed head furnished with both eyes and tentacles, the 

 former situated at the bases of the latter. They have a horny 



