GASTEROPODA. 7 



The Eeteropoda are likewise of very ancient origin, ]ia\dng 

 commenced their existence in the Upper Cambrian deposits. 

 The genera Seller oplion, Cyrtolites, and Maclurea are ex- 

 clusively Palaeozoic ; Bellerophina is found in • the Gault 

 (Secondary), and Carinaria has been detected in the Ter- 

 tiaries. 



The Pulmonate Gasteropoda, as was to be anticipated, are 

 not found abundantly as fossils, occurring chiefly in lacus- 

 trine and estuarine deposits, in which the genera Limnxa, 

 Physa, Ancylus, &c., are amongst those most commonly re- 

 presented. These, however, are entirely Mesozoic and 

 Kainozoic. In the Palaeozoic period the sole known repre- 

 sentatives of the Pulmonifera are the Pupa vetusta, Pupa 

 vermilioncnsis, Dawsonella Meeki, and Zonites priscus of the 

 Carboniferous rocks. 



In the following^ are given the characters of those families 

 of the Gasteropoda which occur in the fossil state, with the 

 leading genera of each family and their range in time. 



Section A. Beanchifera. — Ptespiration aquatic, generally 

 by gills. 



Order I. Peosobranchiata. — Gills situated in advance of 

 the heart. 



Division a. Siphonostomata. — Mouth of the shell notched, or 

 produced into a canal. 



Pam. 1. Steombid^. — Shell with an expanded lip, deeply 

 notched near the canal. Operculum claw-shaped. Foot 

 narrow, adapted for leaping. All the existing genera of the 

 StroiiibidcB are represented in the fossil state, but the family 

 does not seem to have come into existence before the Jurassic 

 period, and it attained its maximum in the Tertiary period. 



The genus Stromhus has a shell with a short spire, a long 

 aperture, and an expanded outer lip, there being a posterior 

 as well as an anterior notch. The Strombs are represented 

 in the Cretaceous and Tertiary rocks, but they attain their 

 maximum in existing seas. 



The Scorpion-shells form the genus Pteroceras (fig. 386), in 



^ In the characters of the families of the Gasteropoda, as ia those of the 

 LavielUbranchiata, Woodward's ' Manual of the Mollusca ' has been mainly 

 followed. 



