DISCOVERY OF THE DISK OF ONYCHOCRINUS 513 



upon the two primitive lines, and not in a parallel progression, it 

 may be assumed to be a subordinate modification, marking the 

 limitations of genera, and perhaps of sub-family divisions. 



4. The difference in general form and habitus, while not explain- 

 able upon any known morphological ground, and therefore with our 

 present knowledge apparently of less value than either of the fore- 

 going, nevertheless furnishes a ground for division which is of some 

 practical importance in the construction of a table, and it may there- 

 fore be given a rank in our classification perhaps higher than it at 

 present logically deserves. 



The other modifications are so palpably limited in their effect 

 upon the history of the group that they need not be considered except 

 in the separation of genera. I do not believe that the higher arm 

 structure is a good character for the definition of families. It appears 

 in parallel successions in other groups of the Crinoids, where it is 

 most interesting in the development from more or less equal branch- 

 ing to radial extensions in the form of main arm-trunks or branches 

 bearing subordinate ramules. This is conspicuously shown among 

 the Camerata in several of the best defined families, viz., in the 

 Rhodocrinidae from Rhodocrinus to Ripidocrinus ; in the Melocrinidae 

 from Glyptocrinus to Melocrinus; in the Actinocrinidae from Actino- 

 crinus to Steganocrinus; in the Platycrinidae from Platycrinus to 

 Eucladocrinus; and in the Hexacrinidae from Arthracantha to 

 Hexacrinus. Yet there can be no thought of questioning the arrange- 

 ment of these beautifully defined families on account of these arm 

 characters, or of contending that they represent anything more than 

 a minor variation. 



The arrangement of which I am at present in search is one for 

 practical utility, that will facilitate the study of this group; and I 

 am not attempting to express fully the phylogenetic relations of the 

 various forms even as I might conceive them to be, although I have 

 tried to recognize some of the evident lines of descent. Taking as 

 a basis the primitive differentiation of the anal system, the Flexibilia 

 Impinnata may be divided into two groups, and the first of these 

 may be again divided upon the interbrachial system and the general 

 form and habitus. This will give three main divisions, A, B, and C; 

 of which A and B will agree with each other and differ from C in 



