DISCOVERY OF THE DISK OF ONYCHOCRINUS 491 



dreds of specimens of wide geographical distribution and great verti- 

 cal range, he will see that there is not the slightest doubt of the validity 

 of the distinction between the two plans of structure. If irregularity 

 in the rays were a prevalent character, we would expect, in species 

 of many genera, numerous variations in the number of primibrachs 

 in different rays of the same individual. But we do not find the 

 number varying indiscriminately among the rays; as a matter of 

 fact, such cases are extremely rare. Besides those above noted, I 

 have seen such variation in scarcely a dozen specimens among all 

 the genera, and these are mostly confined to a single ray. We do not 

 find, to any appreciable extent, intermediate forms — not, indeed, so 

 many as might be expected. Lecanocrinus, and its allied form 

 Pycnosaccus, exhibit some irregularity in a tendency to a single primi- 

 brach instead of two. The few known specimens of Pycnosaccus vary 

 from one to two plates, both in the primibrachs and secundibrachs. 

 The Silurian species of Lecanocrinus, in which the rays are preserved 

 in many specimens, show very few exceptions to the rule of two IBr; 

 but in the Devonian, on the eve of the extinction of the genus, there 

 appears to have been more variability, although our materials for 

 testing it are very meager. Of Schultze's L. roemeri, in the only 

 two specimens I know preserving the arms, the number of IBr varies 

 as follows: No. i, 2-1-1-1-1; No. 2, 2-1-2-2-1. There is also a 

 specimen from the Silurian of Sweden, something like Calpiocrinus, 

 which has one IBr all around except in the posterior ray, where an 

 additional one appears, perhaps a radianal. These cases may be 

 explained by supposing the two IBr to have been coalesced by syzy- 

 gial union, as occurs among the living Comatulae. In Onychocrinus 

 ulrichi there is some variation from the rule of four IBr, some rays 

 having three, and others five. This was at about the extinction of 

 the group, of which Onychocrinus represents the most extravagant 

 development. 



Of course in the Silurian those species with a primitive radianal 

 have a partial equivalent of three IBr in the right posterior ray; but 

 that is constant, and is otherwise accounted for; and it does not 

 affect the rule which prevails throughout the four regular rays. 

 What was the morphological process by which this modification 

 took place, I am unable to explain. The developmental history of 



