512 FRANK SPRINGER 



so probable, that we cannot feel like accepting any scheme which 

 compels their separation. We cannot, of course, represent lines of 

 descent in space of two dimensions, so that anything in the way of 

 a diagram or table would be imperfect, even if we knew all the facts. 

 Still less is it practicable when many of the relationships rest wholly 

 in conjecture. 



In some groups of the Crinoids family divisions are most sharply 

 marked. No one need ever be in doubt, from inspection of the calyx 

 alone, whether a Camerate Crinoid belongs to the Rhodocrinidae, 

 Melocrinidae, Actinocrinidae, Batocrinidae, Platycrinidae, or Hexa- 

 crinidae. This cannot be said of the Flexibilia. By reason of the 

 fundamental difference in construction of the two groups, there is not 

 in the latter that sharp demarkation between calyx and arms which 

 is so characteristic of most of the Camerata. Here, on the contrary, 

 in by far the greater number of the genera, the calyx passes into the 

 arms by imperceptible gradation, so that in the fossil state, being 

 usually unable to see any part of the tegmen, we cannot tell with 

 certainty where the calyx ends and the arms begin. The different 

 modifications of this structure also shade into one another by various 

 transitions, which is the reason why groups of family rank may be 

 formed, as above stated, which differ somewhat according to the 

 character which is taken as the basis of division. 



Nevertheless, it seems possible to form a reasonable opinion as 

 to the relative importance of the characters, as the basis for large 

 divisions : 



1. The differentiation of the anal area, being found completely 

 developed in the earliest Silurian, and continuous almost to the end 

 of its history, may be taken as marking the most primitive division 

 of the group. It evidently dominated the Hnes of descent throughout, 

 and should therefore be accorded first importance in the definition 

 of families, all others being subordinate modifications, affecting one 

 or the other of these lines, but probably not interrupting them. 



2. The presence or absence of interbrachials affords a useful 

 basis of subordinate divisions. 



3. The differentiation of the brachial system in the number of 

 primary brachials, although evidently affording characters of much 

 importance, is one which has impressed itself with varying force 



