THE GASTROPODA i*9 



group of ctenobranchs, the Heteropoda, whose shells are 

 very thin and transparent and symmetrical in their 

 coiling. Both these changes from typical gastropod 

 characters indicate adaptation to a pelagic life that is, 

 one of swimming in the open sea. Transparency is the 

 most effective form of camouflage for a pelagic animal 

 which has nowhere to take cover, and the lop-sided shell, 

 which does very well for a bottom-crawler, would be a 

 serious impediment to active swimming. Heteropoda 

 are very abundant in the Mediterranean and other warm 

 seas ; but fossil examples are rare. The symmetrical 

 character of the shell of Bellerophon led to its being at one 

 time placed among the heteropods, but its shell is much 

 thicker, and the presence of slit indicates its rhipido- 

 glossal affinities. In the Gault occurs a little shell of 

 similar form, Belleropkina, which would have better claim 

 to be considered a heteropod, were it not that it had 

 a nacreous inner layer. 



SUB-CLASS: OPISTHOBRANCHIA. 



The Opisthobranchia are divided into two sub- 

 orders, Tectibranchia and Nudibranchia. The latter are 

 shell-less and unknown fossil. The former is divided 

 into a number of families, but on a basis of shell-form 

 they may be very roughly grouped into (i) more or less 

 turbinate forms, such as Action, distinguished from 

 Streptoneura of similar shape by the strongly- marked 

 folds on the columella, and (2) forms such as Butt a and 

 Scaphander (Fig. 35, /), with a more or less completely con- 



FIG. 35. GASTROPODS (continued). 



(J. Sowerby), Eocene. (Xj.) (After Deshayes). d, Pleurotoma 

 transvenaria Lamarck, Eocene. (X$.) (After Deshayes.) e, Sca- 

 phella lamberti (J. Sowerby), Pliocene. (X|.) (After S. V. Wood.) 

 /, Scaphander edwardsi (J. de C. Sowerby), Eocene. (x.) (After 

 Dixon.) g, Hyolithes princeps Billings, Lower Cambrian. (X.) 

 f, Side view ; g 1 , back view ; g" , operculum. (After Walcott.) 

 A, Limncea fusiformis J. Sowerby, Oligocene. (X f.) (After Edwards. ) 



