GASTEROPODA. 259 



nished with a longitudinal notch or slit. The species of this 

 genus date from the Trias. 



FAM. 1 8. CALYPTR^ID^E : Shell limpet-shaped, with a more 

 or less spiral apex ; interior simple, or divided by a shelly 

 process to which the adductor muscles are attached. Exclud- 

 ing shells whose true position is uncertain, it would appear 

 very questionable if any of the Calyptraida are found in the 

 Palaeozoic Rocks. They are by no means abundant in the 

 Secondary formations ; and though more plentiful in the Ter- 

 tiaries, they attain their maximum in existing seas. 



The genus Calyptrcza includes the so-called " Cup-and- 

 saucer limpets," in which the interior has a half-cup-shaped 

 process attached to the apex of the shell, and open in front. 

 With doubtful exceptions, the fossil species of Calyptrcea are 

 all of Tertiary age. In the genus Crepidula there is a shelly 

 partition covering the posterior half of the interior of the shell. 

 The fossil species date from the Eocene Tertiary. In the 

 genus Pileopsis (or Capuhis) are included the " Bonnet-lim- 

 pets," in which the apex of the shell is spirally recurved. If 

 Platyceras be excluded from this genus, the species of Pileopsis 

 date from the Lias ; but the former is Palaeozoic. 



FAM. 19. PATELLID^:: Shell conical, with the apex turned 

 forwards ; muscular impression horse-shoe shaped, open in 

 front. Foot as large as the margin of the mantle. Respira- 

 tory organ in the form of one or two branchial plumes, 

 lodged in a cervical cavity, or of a series of lamellae sur- 

 rounding 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. 





Fig. 226. MetoptoiiM nycteis. a Side view ; b View of the upper side. 

 Lower Silurian (Billings). 



The genera Patella (including the common Limpets), Ac- 

 mcea, and Metoptoma can hardly be separated in practice 

 from one another. Patella and Acmcea, at any rate, are 

 palaeontologically indivisible, since the only distinctions be- 

 tween them are in the nature of the respiratory organs. In- 



