CLARK: BATHYMETRICAL DISTRIBUTION OF CRINOIDS 133 



• 

 per cent 



Pentacrinitidae 28 



Pentacrinitida 63 



Bourgueticrinidse 82 



Plicatocrinidffi 97 



It is evident from the preceding two tables that the average 

 depth of the habitat of all the genera of any group among the 

 more highly speciahzed types is only a comparatively small 

 fraction of the mean depth of the group as a whole, and that this 

 fraction increases with the age and with the progressive decrease 

 in the specialization of the group, reaching, in the chiefly palaeo- 

 zoic Inadunata, 97 per cent. It is, of course, also high in highly 

 specialized groups confined to a very small range in depth, such 

 as the Perometrinse, Himerometridse and Thysanometrinse. 



Any new or very vigorous group continually gives rise to new 

 forms in the region most favourable for their existence, namely the 

 littoral or subhttoral zones. The existence of a number of such 

 juvenile types within a vigourous group therefore lowers the aver- 

 age depth of the group, which is consequently far less than the 

 mean depth. But in the older mature or senile groups the for- 

 mation of new types in the littoral is inhibited through group 

 senility, and prevented by the occupation of the available eco- 

 nomic territory by types derived from more specialized and 

 more efficient stock. Therefore, theoretically, the older and 

 less specialized the group the closer should the average depth 

 approach the mean. 



In the Atelecrinidse the average depth exceeds the mean depth 

 by 25 per cent; but we know only two genera of this family, one 

 merely from a single specimen of a single species taken only 

 once, in 907 fathoms. This family should therefore be disre- 

 garded in forming general conclusions. 



Putting aside the Atelecrinidse as insufficiently known, it 

 appears to be demonstrable that no crinoid type ever originated 

 in the deep sea, for if such had ever been the case the primarily 

 abyssal types should be unmediately disclosed through showing 

 an average depth of habitat considerably greater than the mean 

 depth. 



The approximation of the average to the mean depth in the 

 older types, taken in connection with what we know in regard 



