MOLLUSCA. GASTEROPODA 253 



(3) Zone of Nullipores or Corallines, from 14 to 

 about 35 fathoms, where the calcareous algae (Corallinacese) 

 are very abundant. It is characterised by the abundance 

 of Pleurotoma, Fusus, Chrysodomus, Buccinum, Natica, 

 Eidima, Venus, Dosinia, Astarte, Nucula, Area, Lima, 

 and Pecten. 



(4) Zone of Brachiop)ods and deep-sea Corals, from 

 35 to about 230 fathoms; off Europe Oculina is the 

 common coral ; Brachiopods and Polyzoa are abundant. 

 Some of the chief molluscs are Turritella, Odostomia, 

 Dentalium, Tellina, Cuspidaria {= Necera), and Yoldia. 



(5) Abyssal Zone, extending from the preceding 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, Nucida, Limopsis, 

 Nuculana, Lima, and Pecten. The remains of Pteropods 

 which have fallen from the surface of the ocean after death 

 are often numerous. 



In the fossil state gasteropods are generally less 

 abundant than lamellibranchs, although they exceed them 

 at the present day. The earliest forms occur in the Lower 

 Cambrian Beds and are referred to the genera Scenella, 

 Stenotheca, Platyceras, and Raphistoma. Throughout the 

 Palaeozoic formations the holostomatous Streptoneura are 

 the predominating forms ; no gasteropods with a well- 

 developed canal are known to occur until the Trias is 

 reached, but siphonostomatous genera become fairly 

 abundant in the Oolites, they increase still more in the 

 Cretaceous, and in the Tertiary they are the principal 

 forms. In the fossil state the other divisions of the 



