INTERRELATIONSHIPS 75 



second the rays and their divisions are rounded exteriorly, and the inter- 

 brachial spaces relatively depressed, so as to emphasize the appearance of 

 divergence above alluded to. 



There is not the slightest difficulty in distinguishing between such forms 

 as Lecanocrinus on the one hand, and Taxocrinus and Onychocrinus on the 

 other, by the above character. But there are occasional species otherwise char- 

 acteristic of the first group, which are rather deeply furrowed between the rays 

 and arms, and some of the second whose arms are habitually rather closely 

 packed together, which we could not so readily assign to their respective 

 groups, except for their evident connection with related genera whose charac- 

 teristic species fall within them without any trouble. There are also a few 

 forms which we are inclined to transfer from the group which they super- 

 ficially resemble, because of some peculiar association of other characters which 

 indicate a probable closer relationship elsewhere. 



Now I confess myself unable to point out any satisfactory morphological 

 basis for the difference in habitus between these two divisions, and I have 

 much doubt as to its structural importance. Yet it is so constant and well 

 marked in many cases, and affords such a palpable and convenient means of 

 separation in this perplexing group, that we find it of some use in our classifi- 

 cations. Both forms existed in the Silurian, and continued into the Car- 

 boniferous; the first one greatly diminished and ending in the Burlington 

 limestone, and the second continuing with increasing importance to the end of 

 the Lower Carboniferous. 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 of small size. In the number of 

 primibrachs and the absence of interbrachials 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 influ- 

 enced the line of succession from the primitive type of this group, and its 

 separation into subordinate divisions. Each 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 pro- 

 duced is a composite, the resultant of the interaction of two or more of these 

 tendencies to modification upon independent lines. Just how much influence 

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

 It is possible to arrange the genera upon the basis of either one of the leading 



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