The Principles of Palaeontology. 197 



have a clearly pelao^ic character, and they must have ulteriorly 

 descended into the depths, as did those which appeared later. 



One of the most interesting characteristics of the Cretaceous 

 epoch is the abundance of siliceous- sponges of the type of the 

 Hexactinellida which now exist in deep water. Close study of 

 the actual distribution of these organisms shows that they occur 

 in very different zones, one of which is not very deep, vas it reaches 

 from 60 to 400 metres. 



§ 3. Influence of the Katuee of Aquatic Conditions, 

 Fresh water facies. — The most precise characteristics of 

 fresh water faunas are furnished by the Mollusks. The genera 

 Paludina^ Bitliynia^ Planorbis^ Limncea^ Physa^ Sioccincea^ among 

 the Gasteropoda ; the genera Unio, Anodonta^ Dreyssentia among 

 the Lamellibranchiata, have been associated in fresh water forma- 

 tions from the beginning of the Jurassic. But to this list must 

 be added also all the terrestrial or aerial animals, whose remains 

 have been carried down by the water courses, or which have been 

 deposited on the shores ; the lacustrine beds of the Tertiary con- 

 tain as much of Helix and Cyclostoma as of Limncea ; in the lacus- 

 trine deposits, also, are found the great majority of the skele- 

 tons of known birds, as well as many reptiles and mammals, 

 which were not necessarily swimming animals. 



Most fossil plants also belong to formations of the same nature ; 

 not only the aquatic species, but all those which grow near 

 water courses, are associated in the same deposits. This is almost 

 the only way in which the purely terrestrial fauna and flora have 

 been, in certain cases, preserved to our day. 



As an example of well known lacustrine formations may be cited 

 the little coal basins of the Central plateau (France); the lake of 

 Commentry with its fauna, its flora and its shores has been, so to 

 say, restored by M. Fayol. At the commencement of the Eocene 

 the lake of Killy occupied a vast area east of Paris ; a watercourse 

 fell there in cascades, and M. Munier-Chalmashas reconstructed all 

 the detai's of that singular locality; plants which love moist 

 places, such as Marchantia^ Asplenium, there covered banks over- 

 shadowed by lindens, laurels, magnolias and palms ; there also 

 were found the vine and the ivy ; mosses {Fontinalis) and 

 Chara sheltered the cray fish {Astacus JEdvmrdsi) Sindi i\iQ 'Edvi- 

 ophthalmas {Heterosphoiroma). Insects and even flowers have left 

 their delicate impressicms in this .travertine. Among other 

 lakes also well known, we may cite the lakes of Armissan 

 and of Aix, at the Oligocene epoch, where the succession of 



