CLASS IV GASTROPODA 



Püeolus Sowb. (Fig. 908). Snicall, cup-shaped to 



Fig. 905. 



535 



conical, ovoid or round. 



Lissochilus sigaretinus Buv. 

 Coral-Rag ; Hoheneggelsen, Han- 

 nover. 



Neritina (iratelonpana Fer. 

 Mioceue ; Häufelburg, near Günz- 

 burg. 



Fig. 908. 

 Püeolus jilicatus Sowb. Bathon- 

 ian ; Langrune, Calvados, ^/i- 



Fig. 906. 



Velates schmidelianus Cham. Lower Eocene 

 (Londinien) ; Cuise-Lamothe, Oise. 



Apex sliglitly cvirved backwards ; only the last wliorl visible. Aperture semicircular ; 

 inner lip broad, callous. Jura to Eocene. 



Order 2. CTBNOBRANCHIATA Schweigger. 



{Pectinihranchia Cuvier ; Azygohranchia von Ihering ; Monotocardia Bouvier.) 



Right cervical gill jJectinate, very large, and usually transposed to the left side, owing 

 to torsion of the hody ; the left gill atrophied. Heart with hut one auricle. Radula 

 smally variously constructed, hut usually armed with fevj teeth in a transverse series. 

 Shell coiled in a more or less elevated spiral, rarely cup- or cap-shaped. 



The Ctenobrancliiata constitute the largest group of the Streptoneura. They are 

 for the most part marine, but some are terrestrial, and some inhabit fresh water. 

 Beginning in the Sikirian, they attain their maximum distribution in the Mesozoic, 

 Tertiary and Recent periods, A division into two groups — Holostomata and Siphono- 

 stomata — according to the nature of the aperture, has been attempted ; but this is 

 unnatural, since it emphasises a shell character whicli is unaccompanied by any 

 anatomical differences. Classifications based upon the structure of the radula, such as 

 have been proposed by Troschel, and more recently by Bouvier, are valueless in 

 Paleontology. Here it will be sufficient to recognise two suborders primarily : 

 Platypoda, in which the foot is typically developed ; and Heteropoda^ in which it is 

 modified into a fin. 



Suborder A. HETEROPODA Lamarck. 



{Nucleohranchiata Blainville.) 



To the Heteropoda belong naked or shell-covered, free-swimming and pelagic 

 marine MoUusks, with distinct liead and highly developed sense organs. Heart, gills, 



