130 CLARK : BATHYMETRICAL DISTRIBUTION OF CRINOIDS 



The families toward the beginning of the Hst are chiefly fami- 

 lies with a comparatively large temperature range; that is, in 

 which one genus has extended itself into water considerably 

 warmer or (usually) colder than the optimum for the family; 

 while those toward the end of the list are largely families which 

 cover a comparatively small temperature range, but which, 

 however, may be confined either to cold or to warm water. 



A family confined either to very cold or to very warm water 

 would be in almost all cases a family of comparatively recent 

 origin, for the coldness of the abysses and the warmth of the tropi- 

 cal littoral are themselves of comparatively recent origin. Also 

 a family with a large temperature range would be of compara- 

 tively recent origin (or at least development) for the temperature 

 of the ancient seas was fairly uniform. 



Thus we should expect the more ancient types to ocfur at or 

 near the centre of our series. 



If we consider the families which are confined to a depth less 

 than 1000 fathoms in contrast to those which occur below 1000 

 fathoms we find that the two groups are as follows: 



Not occurring below 1000 fathoms. Ranging to below 1000 fathoms. 



per cent per cent 



Capillasterinse 23 Zenometrinse 24 



Colobometridse 32 Thalassometrinse 29 



Ptilometrinse 40 Charitometrinae 39 



Calometridae 42 Heliometrinae 39 



Zygometridse 49 Plicatocrinidse 41 



Atelecrinidse 50 Pentacrinitida 47 



Mariametridae 53 Bourgueticrinidse 54 



Himerometridse 54 Bathymetrinae 64 



Antedoninse 58 Pentametrocrinidse 71 



Thysanometrinae 61 



Comactniiinae 62 



Perometrinae 63 



Comasterinae 64 



Average 50 Average 45 



Each group has approximately the same range, and approxi- 

 mately the same average, and neither appears to possess any 

 distinctive characters. 



Similarly if we compare the families the total range of which 



