INTERRELATIONSHIPS JJ 



4. The difference in general form and habitus, while not explainable upon 

 any known morphological ground, and therefore with our present knowledge 

 apparently of less value than either of the foregoing, nevertheless furnishes 

 a ground for division which is of some practical importance in the construction 

 of a table, and it may therefore 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 distal arm structure is a good character 

 for the definition of families. It appears in parallel successions in other groups 

 of the crinoids, from more or less equal branching 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 fami- 

 lies, viz., in the Rhodocrinidae from Rhodocrinus to Rhipidocrinus; in the 

 Melocrinidae from Glyptocrinus to Mclocrinus; in the Actinocrinidae from 

 Actinocrinus 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 arrangement of those clearly 

 defined families on account of such arm characters, or for 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 may be divided into two groups, and the first of these 

 may be again divided upon the infrabasal development, the interbrachial sys- 

 tem, and the general form and habitus. This will give two main divisions, 

 A and B ; and three subdivisions under A. In this way the known genera of 

 the group — with the exception of some transition forms whose place is difficult 

 to assign — will fall into" four family divisions, which differ from one another in 

 various degrees, accordingly as one or the other character is given the greater 

 importance. Each division or family contains further groupings of genera 

 based upon some of the other characters, which might be given subordinate 

 designations according to differing notions of their value. 



This might be considered an imperfect attempt to work out the resultant 

 of the several modifications which I have mentioned, and it necessarily encoun- 

 ters difficulties which can be evaded only by some arbitrary — and perhaps tem- 

 porary — disposition of the disturbing elements. As to these no scheme will 

 ever be perfectly satisfactory, and there will always be some shifting of opin- 



