DISCOVERY OF THE DISK OF ONYCHOCRINUS 511 



a rather too prominent basis of my former arrangement. Both forms 

 existed in the Lower Silurian, and continued into the Carboniferous; 

 the first one greatly diminished and ending in the Keokuk Limestone, 

 with a single exception — probably a transition form — in the Coal 

 Measures; and the second continuing with increasing importance 

 to the end of the Subcarboniferous. The first division comprises a 

 little group of rare genera, mostly confined to the Silurian, but with 

 evident descendants in the Devonian and Carboniferous. They are 

 mostly small, Ichthyocrinus alone occasionally attaining a consider- 

 able size. In the number of primibrachs and the absence of inter- 

 brachials they fall together nicely, and in the structure of the anal 

 side they represent, for the most part, an earlier stage of development 

 of the Sagenocrinus plan than those of the other division. The 

 second division, with the divergent arms, embraces genera of both 

 forms of brachial modification, and also the two leading types of 

 anal structure. It appeared in the Silurian, and steadily increased 

 to the close of the Kaskaskia, where it is represented by its most 

 conspicuous example, Onychocrinus. 



It is evident that most of the modifications above considered have 

 influenced the line of succession from the primitive type of this group, 

 and its separation into subordinate divisions. Each one of them is 

 doubtless a factor entering into the classification that nature has 

 made — though of very different values — and the probability is that 

 every natural division which has been produced is a composite pro- 

 duct, the resultant of the interaction of two or more of these tenden- 

 cies to modification upon independent lines. Just how much influ- 

 ence each has had in fixing the line of succession we have no means 

 iof determining. It is possible to arrange the genera upon the basis of 

 •either one of the leading morphological changes I have mentioned; 

 but whichever is selected for this purpose, we find our arrangement 

 more or less disturbed by some of the others. For example, a fairly 

 satisfactory arrangement could be based upon the modification of 

 the primary brachials, which would correlate quite well with other 

 ■characters, if it were not for the fact that this would throw Sageno- 

 ,crinus and Forhesiocrinus into different families; whereas the con- 

 :nection betewen these genera is so evident, and the line of descent 



