23(i DISTRIBUTION OF THK MOLLUSCA IN TIME. 



decadence ; the gastropods progressing, the lamellibranchs 

 stationaiy. 



" The peculiarly tertiary genera are prett}'^ numerous. Among 

 the cephalopods we cite : Spirulirostra, Scaptorrhynchus, Belop- 

 tera, Belosepia, Yasseuria, Aturia ; among the gastropods : 

 Bifrontia, Borsonia, Cordieria, Volvaria, Deshayesia, Diastoma, 

 Proto, Velainella, Pereirsea, Lesperonia, Yelates, etc. ; among the 

 lamellibranchs : Teredina, Anisodonta, Psathui-a, Grateloupia, 

 Lutetia, Pleurodesma, Pecchiola, Carolia, Tindaria, etc. 



" It is needless to add that the proportion of these lost genera 

 diminishes as we ascend the series of stratified rocks, and that 

 the number of modern types in the same degree increases. 



"It is in accordance with this law, that Deshayes has been 

 able to attempt a first classification of tertiary beds. He called 

 Inferior Tertiary, those which contain but 2 per cent, of living 

 species ; Middle Tertiary, those which contain about 18 per cent, 

 of them ; and Superior or Upper Tertiaiy, those having a pro- 

 portion of about 50 per cent. Lyell created the new names : 

 eocene, miocene and pliocene for the divisions of Deshayes. 

 More recently stratigraphy has permitted a rigorous establish- 

 ment of their reciprocal relations, and the confirmation of the 

 purely palaeontological classification proposed by Deshaj^es. 



Terrestrial and Fluviatile Fossil Mollusks. " The distribution 

 in time of these mollusks is very interesting. They are wanting 

 or extremely rare in the ancient beds and do not assume any 

 importance before the tertiary period. According to Bronn, the 

 principal changes in the exterior conditions of existence, con- 

 sisted in the progressive development of the terrestrial surface, 

 in the subdivision of the primordial universal ocean into the 

 Mediterranean and Caspian Seas, in the elevation of plateaus 

 and of mountain chains. A corresponding change was mani- 

 fested in the organic world. With the first exclusively pelagic 

 and swimming population, became associated a marine population, 

 a littoral and finally a terrestrial one. 



" The first terrestrial mollusks have been discovered in the 

 Carboniferous and singularly resemble living forms. Authors 

 have described a great number of Carboniferous terrestrial and 

 fluviatile shells, but it appears that the supposed Unios are 

 Anthracosia ; the Tichogonia, Avicula ; the (European) Planorbis, 

 Serpula, etc. In America, however, we have undoubted ter- 

 restrial genera in the Carboniferous : Pupa, Strophites (allied to 

 Strophia), Zonites, Dawsonella. 



" In the continental (European) Jurassic formations are cited 

 several Cj^rena, Neritina, Planorbis, Melania, Hydrobia. The 

 fauna of the superior lacustrine beds of the Jurassic (Pm*- 

 beckian) and of the lower Cretaceous (Wealdian) is relatively 

 rather rich. The principal genera are Cyrena, Unio, Melania, 

 Valvata. Hvdrobia, Neritina, Planorbis, Phvsa, Limngea, Auri- 



