42 SMITHSONIAN INSTITUTION 



frequent number. As to forms with two or three IBr, there is a marked suc- 

 cession in geological time which has proved to be of importance in the discrimi- 

 nation of genera. The tendency was toward increase in number, so that while 

 2 IBr was the prevailing plan in the pre-Devonian genera, the 3 IBr plan 

 largely predominated in those of post-Devonian epochs. The first plan includes 

 all the Silurian forms except Meristocrinus, and most of the Devonian; it con- 

 tinued through the Carboniferous to the Kaskaskia in the rare genera 

 Wachsmuthicrinus, Mespilocrinus, Metichthyocrinus, Euryocrinus and Syner- 

 ocrinns, and recurred exceptionally in one species of Forbesiocrinus and in the 

 latest species of Taxocrinus. This plan of primibrachs is the one which pre- 

 vails almost without exception throughout the Camerata; it became predomi- 

 nant in the Mesozoic, and remains so to the present time in the great majority 

 of the Stalked crinoids and comatulids — notable exceptions being Metacrinus, 

 C alamo crinus, and Phrynocrinus. 



The 3 IBr plan, while exceptionally indicated by a few cases in the 

 Silurian and Devonian, became a leading feature in the Carboniferous Flexi- 

 bilia, where it is conspicuous in numerous species of the widely distributed 

 genera Forbesiocrinus and Taxocrinus, from the Waverly to the Kaskaskia; 

 in Parichthyocrinus from the Burlington and Keokuk; and it continued until 

 the culmination of the group in Onycho crinus, some species of which regularly 

 add another primibrach. 



This distinction was first pointed out by me in 1906, 1 and it proved to be a 

 very useful character for separating Silurian from Carboniferous species which 

 had all been described under Taxocrinus; those with 2 IBr became Eutaxo- 

 crinus, extending through the Silurian and Devonian and slightly into the Car- 

 boniferous. The change from 2 to 3 IBr occurred in the Devonian, where we 

 find the two characters commingled in a single species; and where species of 

 constant 3 IBr type begin to appear. It was a decided morphological change, 

 and it would be interesting to know by what process it was effected. 2 



The number of brachials in the higher divisions is quite irregular, but 

 occasionally it can be utilized in regard to the secundibrachs for specific 

 distinctions. 



The articulation upon the distal face of the brachials, and on the proximal 

 face of all but the first, is of a type by itself, different from that of other 

 Paleozoic crinoids; it may be described as an imperfect muscular articulation, 

 with fossae, paired muscles and ligaments, but for the most part without a com- 

 plete transverse ridge. It is a modification of the straight muscular articula- 

 tion of the Poteriocrinidae, which also prevails normally throughout the 



1 Journal of Geology, vol. 14, pp. 487 et seq. 



'' This will be discussed by Mr. Austin H. Clark in the forthcoming part II of his Monograph of the 

 Existing Crinoids. 



