The Principles of Palaeontology. 199 



The earliest deposits are clearly marine (beds with Cerithiicm pic- 

 tuin\ with Bitccinum^ Tapes and Mactra ; they constitute the 

 Samartian stage. Higher (Pontian stage) are found enormous 

 quantities of Congeria or Dreissena and Melanopsis, fresh water 

 forms able to exist in waters slightly salt ; they are associated 

 with Cardiidae oi peculiar genera {Adacna, Monodacna), which 

 have continued to exist in the Caspian sea and the sea of Aral. 



The most abundant representative of the Gasteropods is an 

 aquatic Pulmonate provided with plications, the genus Yalen- 

 €ie7i7iesia, which attains a considerable size. 



These beds show, throughout a considerable extent, a curious 

 mixture of marine types capable of withstanding a diminution of 

 the normal saltness ; and of fresh water iortns capable of adapt- 

 ing themselves to a somewhat salt medium Mammals also are 

 found there, such as Dinotherium and the Mastodon. 



A little later, the waters become less and less salt, and the fresh 

 water types more and more preponderate. The Congerias and 

 Cardium retire into the narrow basins, and almost the entire 

 region is occupied by an immense lagoon, inhabited by Paludinas, 

 Meianopsis, the Unionidse, and other fresh water types. But it 

 is a curious fact that the Unios, Paludinas and Melanopses, living 

 over a very extensive area, assume the angular aspect of marine 

 forms, and acquire plications or tubercules. 



Muddy facias. — Corallic fades. — Let us now return to the 

 clearly marine formations, and see what varieties they present. 

 A factor, almos: as important as the condition of the water 

 holding salts in a state of solution, is the nature of the substances 

 held in sicsjjension ; this, characteristic is naturally connected with 

 the geologic nature of the sediment, and the direction of the cur- 

 rents, b or example, we know that at the present epoch the 

 presence of Mussels on a rock indicates in general, muddy 

 waters, and that many forms which abound not far away in 

 clear waters, vanish as soon as the Mussels appear. 



The nature of the bottom is one of the causes which produce 

 great changes in the facies, and that in localities often quite near 

 each other. The best example can be deduced from the study 

 of coral reefs. This study will demonstrate how an entire and 

 complicated fauna, formed of the most diverse elements, is wholly 

 modified under the influence of the same variations of the 

 medium. 



The conditions which the reef -constructing polyps actually 

 require for their development are well known : These are a mod- 

 erately high temperature, a depth not more than 40 metres 



