The Prustciples of Palaeontox^ogy. 191 



medium, appeared in a slight degree among the progenitors, 

 increase more and more in consequence of the biologic reactions 

 which exist among organisms. 



We may find, for instance, for any one epoch, a littoral, 

 pelagic, fresh water, estilary, lacustrine or corallic facies ; sa 

 also we may fi.nd an arctic, temperate, tropical, etc., facies. We 

 shall briefly investigate what indications Palaeontology furnishes 

 for the determination of these facies ; in other words, we shall 

 show how the study of the associations of fossil forms often 

 enables us to attain a knowledge of the conditions of the 

 surrounding medium. 



§ 2. Influence of the Depth of Seas. 



The distribution of animals in the present seas is classed 

 according to five zones, which are pretty clearly defined.^ 



1. The Littoral zone^ which is covered and uncovered with 

 each tide. 



2. The Zone of Laminarians (0 to 27 or 28 metres). 



3. The Zone of Nullipores and Corallines (calcareous Alg^) 

 (28 to 72 metres), also inhabited by large Gasteropoda. 



4. The Zone of Brachiopoda and Corals (72 to 500 metres), to 

 which descend the Corals such as Dendrophyllia and Oculina, 

 the Echini, such as Spatangus, Brissopsis, etc. 



5. The Abyssal Zone, below 500 metres. 



It is seldom possible to determine with precision these zones 

 at different geologic epochs ; but the indications with which 

 this excellent classification furnishes us may be utilized for the 

 research of the bathymetric facies in geologic deposits. We 

 can, in general, distinguish from this point of view three principal 

 facies: the littoral, the pelagic and the abyssal. 



Littoral facies. — The littoral facies is often indicated by the 

 lithologic nature of the rock; conglomerates and coarse sand- 

 stones, can only be formed in the immediate vicinity of the shore. 

 But the palaeontologic characteristics are also very important in 

 determining ancient sea bottoms. 



An excellent criterion is furnished by the boring shells. It is 

 known that at the present epoch, Lamellibranchs belonging to 

 various groups can pierce the hardest rocks ; these forms exist, 



* Elscher, Manuel de Conchy liologie, cJiajJ. III. 



