DISCOVERY OF THE DISK OF ONYCHOCRINUS 497 



Not only so, but the anal series itself leans to the right, so that the 

 vacant space, or the plated integument if it is preserved, is always 

 widest at the left; and the anal plates are sometimes found firmly 

 cemented by pressure to the side of the right posterior ray, leaving 

 no other plates visible at that side, and giving an appearance of 

 sutural union which must be guarded against in studying these fos- 

 sils. The shape and position of this anal row of plates are subject 

 to much variation, owing to pressure in fossilization, but the dex- 

 trorse asymmetry is almost the invariable rule. There does not 

 seem to be in this form any special equivalent of the anal x. It 

 would naturally be the lowest plate, resting on the posterior basal — 

 the original plate of the Antedon larva, or of our supposed proto- 

 type — but it has been so encroached upon by the growth of peri- 

 some at the sides that it is relatively small, and the plates of the 

 series show only a gradual taper from the proximal end upwards. 



This plan of anal structure began in the Lower Silurian, and is 

 found in the Taxocrinoid genera, with or without a radianal, from 

 Eutaxocrinus and Gnorimocrinus in the Silurian, to Taxocrinus and 

 Onychocrinus in the Carboniferous, surviving until the Kaskaskia, 

 long after the other plan was extinguished. Being thus so charac- 

 teristic of that line of genera, it may well be called the Taxocrinus 

 plan. It has persisted to the present time, being found in a very 

 characteristic form in Thaumatocrinus (Plate IV, Fig. 6). 



2. The second plan is that of a simple extension of the anal sys- 

 tem by the addition of other plates of similar solid nature to that of 

 the original plate, and which are joined by sutural union to the adja- 

 cent plates of the posterior rays, and also with each other; so that, 

 as far as they extend upward, they are incorporated into the calyx 

 walls in the same manner as the plates of the other interbrachial 

 areas. The posterior basal is either simply truncate, followed by 

 ■one plate ; angular, followed by two plates ; or truncate with sloping 

 shoulders, followed by three plates; but in each of these cases all 

 the plates, and others of a similar nature succeeding them, form by 

 sutural union (of course of the loose order characteristic of the 

 whole group) part of the calyx wall. 



This form of anal structure is found from the Silurian to the Car- 

 boniferous. The primitive stage of it is found in such genera as 



