492 FRANK SPRINGER 



the group indicates a tendency to get rid of the extra plate in the 

 right posterior ray, if we have been right in considering that plate — 

 both in this group and in the Inadunata — as a radianal. But the 

 change was not characterized by any indiscriminate individual varia- 

 tions ; it must have taken place directly from one structure to the other, 

 through mutations in species and genera. Out of twenty-six speci- 

 mens of Temnocrinus tuherculatus showing the posterior ray, but a 

 single one fails to show the radianal as I have described it. 



In the Silurian, the plan of two IBr prevailed almost exclusively — 

 only two generic forms, represented by Gnorimocrinus loveni in 

 Sweden, and Taxocrinus orbignii in England, having three IBr 

 throughout. One abnormal specimen of Gnorimocrinus expansus 

 has three IBr in one ray, and one of G. tubuliferus has it in at least 

 three rays. So far as I know, these are all the exceptions in the 

 Silurian. The change was more notable in the Devonian, where 

 some species have both two and three IBr and about one-third of the 

 species heretofore referred to Taxocrinus appear with three IBr 

 throughout. The Taxocrinus with two IBr persisted into the base 

 of the Carboniferous with one or two species in the Kinderhook and 

 Waverly; but from there on the three IBr structure became universal 

 in this and related genera. The few exceptional cases we know 

 in individual specimens do not fill the gap between the two struct- 

 ures. It was a decided morphological change, affecting the entire 

 brachial system, and as to the Taxocrini it took place chiefly in the 

 Devonian. 



It follows from these considerations that the Silurian and most of 

 the Devonian species of Taxocrinus should be separated generically 

 from those of the Carboniferous. Since T. tuherculatus has been 

 removed to form the type of Temnocrinus, on account of the posses- 

 sion of a radianal, there remain three species originally referred to 

 the genus by Phillips: T. nohilis, T. macrodactylus, and T. egertoni. 

 In discussing them under his first name, Isocrinus,^ he says that T. 

 nohilis and T. macrodactylus have four "costals." It is clear that 

 he includes what we now call radials and primibrachs, for on p. 29 

 he says: "the pentagonal columnar joint is surmounted by five plates 

 (the pelvis of Miller), alternating with which, and above them, are 



^ Pal. Foss. Cornwall, p. 30. 



