INTERRELATIONSHIPS 65 



as that of the Taxocrinus plan. If the posterior basal is angular above, the 

 face sloping to the right is generally the longest, and the series of plates fol- 

 lowing it a little the largest (PI. XXVII, fig. 8) ; if it is truncate and followed 

 by one large plate which is angular above, the same remark may apply to the 

 second plate. Among such genera as Calpiocrinus, Temnocrinus, and Sagen- 

 ocrinus there are considerable variations of this plan; but throughout its entire 

 range — except in certain peculiar transition cases to which I will allude later — 

 there is no suggestion, in the external form of the cup, of any such structure 

 as an anal tube. 



Under this form, as well as the first, the modification reached the phase 

 of the entire disappearance of the anal plates, first in the Silurian without 

 disturbing the radianal, as in Ichthyocrinus, and afterwards in the Carbonifer- 

 ous with complete elimination of that plate, as in Metichthyocrinus. 



Looking at these well-marked examples of the two plans, one cannot fail 

 to be impressed with their complete distinctness as they stand side by side in 

 the Silurian (Pis. XVIII and XLVII), and in the Carboniferous (Pis. XXIX 

 and LIII). They run a somewhat parallel course during the greater part of 

 the paleontological history of the group. The Taxocrinus plan, beginning first, 

 maintains a vigorous course to its culmination in the Kaskaskia in the genera 

 Taxocrinus and Onychocrinus ; while the Sagenocrinus plan, starting later, 

 culminated in the Keokuk and Warsaw groups, although continuing with a 

 degenerate form into the Coal Measures. The former is thus the one charac- 

 ter connected with the anal structures which survived from the Ordovician 

 almost to the extinction of the group in the Carboniferous, and is still repre- 

 sented in the larvae of the existing forms. 



Notwithstanding the evident distinctness of the two plans, their early 

 divergence from a common origin which can with reasonable probability be 

 inferred, and their long duration as independent lines of structure, it is never- 

 theless a curious fact that they also tend to run together, in a sort of conver- 

 gent evolution, toward the close of their history. I can show most beautifully 

 by actual specimens how this has occurred in the Carboniferous between 

 Forbesiocrinus and Taxocrinus, not so much in the way of individual varia- 

 tions as in the modification of species. We have a species of Forbesiocrinus 

 in the Keokuk limestone with a firmly plated anal area, in which there is a well- 

 marked vertical series imbedded in the middle, but usually starting on the 

 longer right shoulder of the basal (PI. XXVIII, figs. 4, 5). On the other hand, 

 there is in the same formation, and at about the same horizon, a form of 

 Taxocrinus in which, while the anal series is rounded and prominent, and 

 merges plainly into the perisome above, the plates of the bordering integument, 

 though still movable, are strong and heavy, and apparently united to the ad- 

 jacent brachials by loose suture (PI. LIX, figs. 2, 3, 4). In this the modifica- 



