CLASS IV ' GASTROPODA 535 



Pileolus Sowh. (Fig. 908). Small, cup-shaped to depressed conical, ovoid or round. 







Fig. 905. 



Lissochilus sigaretinus Buv. 

 Coral-Rag ; Hoheneggelsen, Han- 

 nover. 



Fio. DOT. 



Neritina gratcloupcma Per. 

 Miocene ; Haufelburg, near Gunz- 

 burg. 



Fig. 90S. 



Pileolus plicahis Sowb. Bathon- 

 iaii ; Langrune, Calvados. 3/i. 



Velates sclimiilelianus Chem. Lower Eocene 

 (Londinlen) ; Cuise-Lamothe, Oise. 



Apex slightly curved backwards ; only the last Avhorl visible, 

 inner lip broad, callous. Jura to Eocene. 



Aperture semicircular; 



Order 2. CTENOBRANCHIATA Schweigger. 



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



Bight cervical gill fectinate, very large, and usually transposed to the left side, Giving 

 to torsion of the body ; the left gill atrophied. Heart with hut one a,uricle. Eadula 

 small, variously constructed, hut usually armed loith fev} teeth in a transverse series. 

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



The Ctenobranchiata 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 Silurian, 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 

 iTunatural, since it emphasises a shell character which is unaccomj^anied by any 

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

 have been j)roposed 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 Mollusks, with distinct head and highly developed sense organs. Heart, gills, 



