CLARK: BATHYMETRICAL DISTRIBUTION OF CRINOIDS 131 



is less than 1000 fathoms with those in which it is greater we find 

 that neither group possesses any marked characteristics. 



Ranging more than 1000 fathoms. Ranging less than 1000 fathoms. 



per cent per cent 



Thalassometrinae 29 Capillasterinse 23 



Charitometridse 39 Comactiniinse 62 



Zenometrinse 24 Comasterinse 64 



Heliometrinse 39 Zygometridse 49 



Bathymetrinse 64 Himerometridse 54 



Pentametrocrinidse 71 Mariametridse 53 



Pentacrinitida 47 Colobometridae 32 



Bourgueticrinidse 54 Calometridse 42 



Plicatocrinidae 41 Ptilometrinse 40 



Antedoninse 58 



Thysanometrinse 61 



Perometrinse 63 



Atelecrinidse 50 



Average 45 Average 50 



But if we consider the larger groups we are at once struck 

 by the fact that of two comparable types the more highly spe- 

 cialized always shows a much lower figure than the more primi- 

 tive, the figure for the former being only from 28 per cent to 41 

 per cent (with the average 35 per cent, or little more than one- 

 third) of the figure for the latter. In the following list four 

 strictly comparable groups are given: 



More specialized More primitive 



per cent per cent per cent 



Articulata 14 Inadunata 41 (34) 



Pentacrinitidse 15 Bourgueticrinidse. . . 54 (28) 



Comatulida 15 Pentacrinitida 47 (32) 



Oligophreata , . . 10 Macrophreata 24 (41) 



This is due to the great predominance of the more specialized 

 and more recent types in the warm shallow water, from which 

 they have not as yet spread into the deep sea, but from which, 

 through their superior economic equipment, they have to a 

 greater or lesser extent extirpated the preceding less specialized 

 and more ancient forms which, persisting chiefly, therefore, in 

 the deeper and consequently cooler and more uniform water, 

 naturally show much greater generic ranges, and consequently 

 higher percentages representing the mean range as compared 

 with the total range of any one family. 



