34 GASTEROPODA. 



iu a cervical cavity, or of a series of lamellte surrounding the 

 animal between the body and the mantle. The Patellidce 

 commence to be represented in the Lower Silurian rocks, 

 and have continued to the present day. 



The genera Patella (including the common Limpets), 

 Acmcea, and Metoptoma can with difficulty be separated in 

 practice from one another by theu' shells alone. Patella 

 (fig. 419, g) and Acmcea, at any rate, are palseontologically 

 indivisible, since the only distinctions between them are in 

 the nature of the respiratory organs. Including these gen- 

 era, therefore, in one, the range of the Limpets is from 

 the Silurian upwards. 



Some of the Upper Cambrian Limpets have been separated 

 under the name of Palaemcea, but the differences between 

 these and Patella proper appear to be slight. 



The genus Metoptoma (figs. 419, f, and 422) very closely 



Fig. 422. — Metoptuma nycteis. a, Side view ; 6, View of the upper side. L^jwer Silurian. 



(BUlings.) 



resembles Patella, but tlie muscular scar consists of a number 

 of disconnected 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 abun- 

 dant in the Lower Silurian series ; but they range as far as 

 the Carboniferous. It may be doubted, how^ever, if some of 

 the so-called Mctoiitomce are not really the posterior plates of 

 Chitons. 



Fam. 20. Dextalid.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 

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

 comprises the principal genus Dcntalium, in the wide sense 



