5i6 FRANK SPRINGER 



that the most important characters often shade into one another to 

 a greater or less degree. For instance, in speaking of the arm- 

 branching, we cannot confine the term " dichotomous " to such sym- 

 metrically dividing rays as are found in Cyathocrinus; but it must 

 be taken to mean simply that the rays divide by a more or less regular 

 bifurcation, as opposed to those in which the branches are given off 

 in the form of lateral ramules markedly smaller than the main trunk, 

 as in Onychocrinus, which we call "heterotomous." So as to whether 

 the arms are contiguous or divergent, the terms are relative and not 

 absolute; and while, as in other characters, the extremes are well 

 marked, the two plans of arm arrangement sometimes approach each 

 other so closely that all we can say in some genera is that the tendency 

 is in one direction rather than the other. In the matter of interbrach- 

 ials, there are really two kinds of plates: the regular, strong plates 

 that occur in the axils, and the small irregular plates or granules 

 forming the pliant integument which extends from the tegmen, in 

 varying degrees, and for varying distances, down between the rays 

 and their divisions. When in the descriptions we speak of "IBr 

 plates, " we mean the former. 



It must also be remembered that it is impossible in any such 

 scheme to represent the exact degree of relationship of genera, even 

 as we understand it to be. We necessarily grade our divisions accord- 

 ing to the greater or less generality of the characters; and we find 

 in some cases that this will throw into different larger groups two 

 genera that we should upon other grounds place next to each other. 



Bearing in mind these qualifications, the following analysis may 

 be found fairly practical, according to our present knowledge: 



ANALYSIS OF THE GENERA 



I. Anal plates, when present, incorporated in calyx by sutural union 

 with adjacent rays. 

 A. Rays in contact all around, or separated only at anal side; arms 



contiguous . ICHTHYOCRINIDAE 



a. Radianal. 



i. RA under r. post. R. 

 Primibrachs 2. 



Arms dichotomous. 



Anal X alone, or followed by others .... Clidochirus 



No anals Ichthyocrinus 



Arms heterotomous 



