The PKmciPLES of Palaeontology. 193 



from animals which lived either at the surface or at various 

 depths. 



The expression Pelagic fades ^ in Geology, should not be under- 

 stood in the same sense; there are no anciert deposits in which 

 the fauna consists exclusively of swimming animals. A pelagic 

 fauna at the present day might inhabit seas whose bottoms are 

 very variable in depth ; it might reach to the vicinity of the 

 C( asts or, again, might extend over the deep abysses ; it is evident 

 that in geologic deposits such a fauna might be associated with 

 the elements of all the zones. 



The idea then of the pelagic facies is broadened, and we refer to 

 this facies the deposits formed in the deep sea, at a considerable 

 distance from the shore, but not in the abysmal depths ; they 

 are characterized by the absence of the elements of the fauna of 

 the littoral zone, and by a mixture of forms adapted to swim- 

 ming, such as Cephalopods, Pteropods, Fishes, with creeping or 

 fixed forms (Echinoderms, Brachiopods, Gasteropods, Lamelli- 

 branchs), the species of which differ from those living near the 

 shores. 



Pelagic forms, well adapted for swimming, have naturally an 

 area of distribution much more extensive than the littoral forms, 

 and very much more so than the fixed and sedentary forms. 

 This results not only from the fact that these animals being able 

 to traverse large areas become distributed more easily, but also 

 from the fact that the seaward c editions are much more con- 

 stant than those near the shores. Thus it is that the species of 

 Ammonites and Belemnites, in the secondary epoch, are found at 

 the same horizon in places far distant from each other, and con- 

 sequently hold an important place in the chronologic correlation of 

 deposits. The European and American forms are almost identi- 

 cal, whilst the reverse occurs as regards the Lamellibranchs of 

 the same epoch. 



Abyssal facies. — Submarine explorations which have every- 

 where been matters of much attention, have resulted in clearing up 

 the mystery of the conditions of life in the great depths.* It is 

 now known that there exists at the bottom of the sea a very rich 

 fauna, with special characteristics, but without close relations to 



* Reports of the Challenger Expedition ; Narrative of the Cruise. E. Perrier, Les Explorations 

 aous-marines. 1886, chap. VIII. 



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