DISCOVERY OF THE DISK OF ONYCHOCRINUS 509 



inside they appear as mere points at the outer surface, and often 

 they do not pass through the wall; so that in a given interradius 

 we may count twice as many plates interiorly as exteriorly. In some 

 genera which are usually without any such plates, a straggling one 

 is occasionally seen, and it is quite probable that, if we could see 

 the interior of all the specimens, we should find many instances of 

 such plates which have not come to the surface. It is a fact thoroughly 

 established that these plates multiply with the growth of the indi- 

 vidual; but it would seem that in those forms in which the rays are 

 more or less contiguous, and tend to arch over the interbrachial 

 areas, the plates are crowded by the growth of brachials, and to a 

 certain extent reduced or suppressed at the outer surface of the calyx. 

 In genera like Taxocrinus, Forhesiocrinus, and Uintacrinus the 

 variance of the interbrachials with age is very marked, young indi- 

 viduals in some species having none at all, while the adults are pro- 

 fusely supplied. In Wacksmuthicrinus , which has no anal plate, the 

 interbrachials are still more variable, being present or absent in the 

 adult of the same species. In Nipterocrinus, and probably in 

 Pycnosaccus, and the form described by Angelin as Forhesiocrinus 

 obesus, the perisome extended down to the radials as a plated inte- 

 gument, without developing any well-defined interbrachial plates. 



Notwithstanding the irregularities in some cases above mentioned, 

 the varying development of the interbrachial plates aftords important 

 characters for classification in this group. In some genera the peri- 

 some did not extend down between the rays in such a manner as to 

 form any permanent plates in the interbrachial areas; in others they 

 were abundantly developed. Both forms extend from the Silurian 

 to the Carboniferous; the first being characteristic of a little group 

 of genera which may be taken as the typical Ichthyocrinidae, and 

 correlating with another character to be mentioned presently. The 

 tendency in the paleontol''gical history of the group generally is the 

 same as in the individual, viz., toward an increase in interbrachials. 

 But few forms without such plates are found after the Silurian, 

 while in the Carboniferous their presence in considerable numbers 

 is the general rule. Nevertheless, the latter stage was fully attained 

 in the Silurian in the genus Sapenocrinus. 



