260 MOLLUSCA. 



eluding these genera, therefore, in one, the range of the 

 Limpets is from the Silurian upwards. 



The genus Metoptoma (fig. 226) very closely resembles 

 Patella, but the muscular scar consists of a number of dis- 

 connected cavities. In the typical species, also, the anterior 

 side, under the apex of the shell, is truncated or nearly straight. 

 Species of Metoptoma are particularly abundant in the Lower 

 Silurian series ; but they range as far as the Carboniferous. 



FAM. 20. DENTALID^E : Shell tubular, symmetrical, curved, 

 open at both ends. Aperture circular. Foot pointed, with 

 symmetrical side-lobes. The " Tooth-shells " are generally 

 placed here, in the vicinity of the Limpets ; but they are re- 

 ferred by Huxley to the class of the Pteropoda. The family 

 comprises the single genus Dentalium, well known by the 

 tubular, smooth, or longitudinally striated shell, open at both 

 ends. 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 

 Devonian, and more especially from the Carboniferous Rocks, 

 some of them of large size ; but more or less doubt obtains as 

 to the true nature of these. The Secondary Rocks have 

 yielded a considerable number of species, and they become 

 still more numerous in the Tertiaries. 



FAM. 21. CHITONID^E: Shell multivalve, composed of eight 

 transverse plates, disposed one behind the other in an imbricat- 

 ed manner. Animal with a broad creeping foot; branchiae 

 forming a series of lamellae between the foot and the mantle, 

 round the posterior part of the body. The Chitonida com- 

 prise 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, attaining their maximum at the pre- 

 sent day. 



The distinctive peculiarities of the shell of the 

 Chitons (fig. 227), by which they may always be 

 separated from the Cirripedes, are the following : 

 i. The shell never consists of more or fewer than 

 eight pieces. 2. The valves of the shell are al- 

 ways 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 dis- 

 tinct areas a dorsal and two lateral areas. 4. Each plate 

 is imbedded in the mantle of the animal by forward extensions 

 of its front edge, which are termed the "apophyses." 



The Chitons are represented by fossil species in the Silurian, 



