GASTEEOPODA. 



35 



of this name, well known by the tubular, smooth, or longi- 

 tudmally-striated shell, open at both ends (fig. 423). The 

 fossil species are liable to be confounded with the tubes of 

 Tubicolar Annelides, or a reverse mistake to this may be 

 made. Several species have been described from the Devo- 

 nian, and more especially from the Carbonifer- 

 ous rocks, some of them of large size ; but more 

 or less doubt obtains as to the true nature of 

 some of these. The Secondary rocks have 

 yielded a considerable number of species, and 

 they become still more numerous in the Ter- 

 tiaries. It does not seem impossible that some 

 of the so-called forms of Theca {Hyolithcs), from 

 the Silurian and Upper Cambrian, may really 

 prove to be referable to Dcntalium. The genus 

 Gadus, ranging from the Cretaceous to the 

 present day, is separable from Dentalium chiefly 

 by the characters of the animal itself, but tlie 

 shell in the former is small, and has a contracted 

 anterior extremity and a polished surface. 



Eam. 21. Chitonid.'E. — Shell multivalve, 

 composed of eight transverse plates, disposed 

 one behind the other in an imbricated manner. 

 Animal with a broad creeping foot ; branchiae forming a 

 series of lamellse between the foot and the mantle, round 

 the posterior part of the body. The Chito- 

 nidce comprise only the single genus Chiton, 

 with several more or less distinct sub-genera. 

 The species of the family commence in the 

 Lower Silurian, and are rare as fossils, attain- 

 ing their maximum at the present day. 



The distinctive peculiarities of the shell of 

 the Chitons (fig. 424), by which they may 

 always be separated from the Cirripedes, are 

 the following : 1 . The shell never consists of 

 more or fewer than eight pieces. 2. The valves 

 of the shell are always placed one behind the other in a 

 unilinear series. 3. The six middle plates of the shell are 

 divided, each, by lines of sculpturing into three distinct areas 



Fig. 423.— Den- 

 taliuvi Bo^ieU 

 Miocene. (After 

 Desliayes.) 



Fig. 424. — He;- 

 minthochiton Grif- 

 fithli. Silurian. 

 (After Salter.) 



