18 THREE CRUISES OF THE " BLAKE. 



5) 



Many groups are remarkable for the variety of their forms, so 

 that it is ahuost impossible to apply to them any classification, 

 even that regarded as best established. From the study of these 

 groups, most interesting morphological and palaeontological re- 

 sults have been derived. Some of these are discussed in con- 

 nection with the account of the different zoological groups. As 

 the corals of the West Indies have been carefully studied by 

 Pourtales, we may dwell more at length on the relations of that 

 fauna to their precursors in the tertiary period. 



The corals of the European tertiaries are so well known from 

 the works of Milne-Edwards, Haime, Reuss, Seguenza, Duncan, 

 and others, that we can compare the living West Indian coral 

 faunae, both littoral and abyssal, with that of the European 

 tertiaries. The resemblance is a striking one, and we may 

 safely, from analogy, reconstruct the physical conditions which 

 existed in the European tertiary seas, and picture to ourselves the 

 depth of the water, the purity of the sea, and the intense aera- 

 tion of the waters, far from great bodies of fresh water, which 

 must have prevailed in those days over areas where either coral 

 reefs or a deep-water fauna flourished.^ 



Fewer deep-sea genera are common to the tertiary and living 

 faunse of the West Indies than to the European tertiary and the 

 living West Indian fauna. This may be due to smaller changes 

 of level in the latter region than in Europe. Yet if we take into 

 account the fact that the numerous West Indian extinct genera 

 belong to families of deep-sea corals, we may safely conclude 

 that there have really been important changes of level in the 

 West Indian area. The presence of European cretaceous fossils 



^ The similarity in the deep-water rence of the recent stalked crinoids in 



types and their fossil representatives may snch deep water as compared with that of 



not invariably mean existence under iden- the palfeozoic period may be interpreted 



tical conditions. We have the most sat- to represent the conditions necessary for 



isfactory evidence that the crinoids of the the maintenance of the type down to the 



silnrian deposits of the State of New present day. In the present epoch depth 



York flourished in shoal-like areas, and represents, as has been suggested by 



that during the Jurassic period their oc- Pourtales, the great pressure to which 



currence on the coral reefs of that time the heavy atmospheres of earlier periods 



showed these ancient crinoids to have subjected the animals of those days, and 



lived in much shallower waters than their thus perpetuates conditions recalling 



recent allies, the Pentacrinus and Rhizo- those of the shoal waters of early ages, 

 criuus of the West Indies. The occur- 



