166 MOLLUSCA. GASTEROPODA. 



Some of the chief mollusks are Turritella, Odostomia, 

 Dentalium, Tellina, Necera, and Yoldia. 



(5) Abysmal Zone, extending from the last down to 

 the greatest depths at which life has been found. In this 

 zone the shells are mostly thin, colourless, transparent and 

 of small size ; it is especially characterised by numerous 

 Scaphopods ; other common forms are Pleurotoma, Fusus, 

 Actceon, Scaphander, Philine, Area, Nucula, Limopsis, 

 Leda, Lima, and Pecten. The remains of Pteropods which 

 have fallen from the surface of the ocean after death are 

 also numerous. 



In the fossil state gasteropods are less numerous than 

 lamellibranchs, although they exceed them at the present 

 day. The earliest forms occur in the Upper Cambrian, 

 they belong to the holostomatous Prosobranchiata, and 

 throughout the Palaeozoic this is the predominant group, 

 no genera belonging to the siphonostomatous section are 

 known with certainty until the Trias is reached ; the latter 

 become more abundant in the Oolites, still more so in the 

 Cretaceous, and in the Tertiary they are the predominant 

 forms. The other orders are not nearly so well represented 

 as the Prosobranchiata. The Polyplacophora range from 

 the Ordovician to the present day, but they are more 

 abundant in the Palaeozoic than in later formations. The 

 Heteropoda are represented by a few forms only, the first 

 occurring in the Miocene. The Opisthobranchiata range 

 from the Carboniferous to the present day, they are 

 moderately well represented in the Jurassic and Cretaceous 

 and become more abundant in the Tertiary. The Pul- 

 monata are first found in the Carboniferous, but they are 

 quite rare until the Tertiary is reached. The Pteropoda 

 occur first in the Lower Cambrian, but the nature of some 

 of the earlier forms (Hyolithes, Conularia, Tentaculites) is 



