No. 514] 



ARTICULATIONS OF CRINOIDS 



585 



ment. The similarity is even more striking; in tin 1 

 urchins new plates are added only between the ocular and 

 the next succeeding plate, and it has been urged that tie' 

 addition of new plates only at the ends of the arms in 

 crinoids constitutes an important morphological differ- 

 ence between the ambulacral systems of the two groups. 

 I have recently shown, however, that in almost all the 

 recent crinoids (and similarly in many of the fossils) 

 plates are added between the radials and next following 

 joints (interpolated division series) just as in the urchins; 

 the process of interpolation is different, but the result is 

 morphologically identical in the two cases. J. S. Miller 

 in 1821 first called attention to the similarity of a crinoid 

 to an inverted Cidaris, but I believe the resemblance be- 

 tween the ambulacral systems of the two are closer and 

 more fundamental than was supposed either by Loven 

 or by Carpenter. All the recent and most of the fossil 

 crinoids have uniserial arms, but the brachials are always, 

 at least in the proximal third of the arm, triangular or 

 obliquely wedge shaped, a condition which is most pro- 

 nounced in the young. Now, applying Jackson's law <»! 

 "localized stages," we may assume in the crinoid arm 

 that the joints which are ontogenetically the oldest are 

 phylogenetically the oldest also; and we should thus be 

 led to look for ancestral characters toward the arm bases. 

 Here we find joints much more triangular than farther 

 out, the distal and proximal ends being very oblique; 

 hence we should infer an ancestry of forms with sharply 

 triangular brachials ; but certain comatulids go even far- 

 ther in the proximal third of the arm, the braebials hav- 

 ing borders so oblique that the inner apices of the tri- 

 angles do not reach to the opposite border of the arm. 

 Judging from recent forms alone, then (to say nothing 

 of the fossils), we are irresistibly led to the conclusion 

 that the biserial condition is the fundamental condition 

 of the crinoid arm, just as the double row of ambulacral 

 plates is the fundamental condition in the urchins, and 

 that the monoserial arrangement is purely secondary, 



