304 



HISTORICAL PALEONTOLOGY. 



found in warm regions, inhabiting fresh or brackish waters, 

 undergoes a vast development in the Eocene period, where it 

 is represented by an immense number of specific forms, some 

 of which attain very large dimensions. In the Eocene strata 

 of the Paris basin alone, nearly one hundred 

 and fifty species of this genus have been 

 detected. The more strictly fresh-water 

 deposits of the Eocene period have also 

 yielded numerous remains of Univalves such 

 as are now proper to rivers and lakes, to- 

 gether with the shells of true Land-snails. 

 Amongst these may be mentioned numerous 

 species of Limnaa (fig. 220), Physa (fig. 221), 

 Melania, Paludina, Planorbis, Helix, Buli- 

 mus, and Cyclostoma (fig. 222). 



With regard to the Cephalopods, the chief 

 point to be noticed is, that all the beautiful 

 and complex forms which peculiarly char- 

 Fig. 219. Centhi- acterized the Cretaceous period have here 



um hexagenum. Eo- 

 cene, disappeared. We no longer meet with a 



single example of the Turrilite, the Baculite, 

 the Hamite, the Scaphite, or the Ammonite. The only ex- 

 ception to this statement is the occurrence of one species 

 of Ammonite in the so-called "Lignitic Formation" of North 

 America; but the beds containing this may possibly be rather 

 referable to the Cretaceous and this exception does not 

 affect the fact that the Ammonitida, as a family, had be- 



Fig. 220. Limncea 

 pyramidalia. Eocene. 



Fig. 221. Physa 

 columnaris. Eocene. 



Fig. 222. Cyclostoma 

 Arnoudii. Eocene. 



come extinct before the Eocene strata were deposited. The 

 ancient genus Nautilus still survives, the sole representative of 

 the once mighty order of the Tetrabranchiate Cephalopods. 



