DISTRIBUTION OF THE MOLLUSCA IN TIMK. 235 



appears in the Cenomanian, and its existence is sliort. The 

 cretaceous gastropods are relatively rare and belong to the 

 genera Scalaria, Turritella, Chemuitzia, Nerinaea, Avellana, 

 Globiconcha, Varigera, Pterodonta, Natica, Trochus, Turbo, 

 Pleurotomaria, Chenopus, Anchura, Voluta, Fusus, Mitra, 

 Columbellina, Pleurotoma, Pyrula ; consequent!}^ the Siphon- 

 ostomata commenced to develop. * The lamellibranchiates 

 approached closely to living types ; but several forms are lost ; 

 Inoceramus, Myoconcha, Isoarca, Opis, Thetis, and the entire 

 series of the Rudistes (Caprina, Caprinella, Caprotina, Radio- 

 lites, Hippurites), which expired with the chalk, and the different 

 horizons of which are so important to geologists. 



Neozoic Fosiiil Mollusks. " Tertiary fossils nearly all belong 

 to living types. The Ammonites, Belemnites, Nerinseas, and 

 Rudistes have become extinct. 



"In 1855, Bronn knew more than 8000 species of tertiary 

 mollusks, thus distributed : — 



Brachiopods, . . . 52 Heteropods, Pteropods, 25 

 Lamellibranchs, . 2445 Cephalopods, ... 37 

 Gastropods, . . 5310 

 " The gastropods are dominant, then come the lamellibranchs. 

 The cephalopods ai-e completely in decadence and numericall}'- 

 inferior to the brachiopods. Consequently, the tertiary period 

 was the period of gastropods, a domination which continues to 

 the present time. 



" In fact, if we compare the tertiary fauna of the Paris basin 

 with the living fauna of the French coast we find the following 

 confirmatory figures : — 



Basin of Paris Coast of France 



(Deshayes). (Fischer). 



Brachiopods, sp., 15 8 



Lamellibranchs, sp., .... 1026 1*76 



Gastropods, sp., 1836 ' 364 



Cephalopods, sp., 14 21 



" The relative importance of the types of Malacozoa in the 

 three great periods may be thus represented : — 



Palaeozoic. Mesozoic. Neozoic. 



Brachiopods, 1 4 3 



Lamellibranchs, 2 1 2 



Gastropods, 4 2 1 



Cephalopods, . 3 3 4 



" Thus the brachiopods and cephalopods are to-daj'^ plainly in 



* Dillwyn has observed that the shells of carnivorous gastropods were 

 almost or entirely wanting in the palaeozoic and secondary strata ; but 

 they were then replaced, in the economy of nature, by the now almost 

 extinct order of teti-abraucliiate cephalopods — of which several thousand 

 species have been described. 



