NO. 7 SEA-LILIES AND FEATHER-STARS — CLARK I5 



THE COMPOSITION OF THE CRINOID SKELETON 

 The skeleton of the crinoids has the composition of a moderately 

 magnesian Hmestone, the proportion of magnesium carbonate to 

 calcium carbonate appearing to be a function of temperature and 

 rising from 7.26 per cent in the coldest water to as high as 13.74 per 

 cent in the tropical littoral. A trace of phosphate of lime appears 

 always to be present, but whether or not it is an essential constituent 

 is uncertain. 



THE DISTRIBUTION OF THE CRINOIDS 



The modern distribution of the crinoids has been chiefly the result 

 of the gradual differentiation in the conditions of dift"erent sections of 

 a once uniform sea area resulting in the evolution of more or less 

 distinct faunal units through the selective extirpation of dift'erent 

 types in different regions; combined with this there has been under 

 the changing conditions the evolution of a few new types and the 

 further specialization of others, some of which, efficient and aggres- 

 sive, have apparently extirpated the previous crinoidal inhabitants 

 from all the regions into which they have been able to extend their 

 range. 



Changing geological and meteorological conditions affect chiefly the 

 shallow water of the littoral and sublittoral regions ; the intermediate 

 depths are but slightly affected, and the abysses not at all. Thus 

 faunal differentiation is most marked in shallow water, much less 

 marked in water of intermediate depth, and but vaguely indicated 

 in the abysses. 



One of the results of the gradual change in conditions in a once 

 nearly uniform ocean has been the discontinuous distribution of many 

 marine types occurring in two or more widely separated localities 

 which have been subjected to less modification than the intermediate 

 regions but which are now separated by impassable thermal or other 

 barriers. This is well brought out in many marine types and in the 

 crinoids is illustrated by the curious correspondence between the 

 fauna of Australia and that of the Caribbean Sea. 



After the differentiation of a fauna land barriers sometimes appear 

 isolating a certain section from the main range. It is thus that we 

 account for the reappearance of Arctic types in the Okhotsk and 

 Japanese seas. 



Geographically the littoral crinoids are divisible into a number of 

 more or less well-marked faunas corresponding in the main to those 

 indicated by other marine types. In those regions where the tem- 

 perature of the water is more or less uniform from the surface to 



