242 PROCEEDINGS OF THE ACADEMY OP [1879. 



nus, there are but three unequal plates in the proximal ring, show- 

 ing here an actual metamorphosis of five plates into three. 



In Cupressocrinus, which has a single central plate below the 

 " subradials," pierced by the quadripartite perforation of the 

 column, it might at first seem difficult to subdivide the proximal 

 plate in this manner. But it will be noted that it is really radial 

 in position, since its five external angles alternate with those of 

 the "subradials," and furthermore, we find that nature has done 

 it for us in a precisely similar case. Myrlillocrinus, whose base is 

 perforated by a quadrangular foramen, has its proximal ring 

 divided into five small pieces, alternating with "subradials" and 

 hence radial in position, (loth New York Regents' Rep., p. 142.) 



It is worthy of note that in the form last discussed those with 

 two rings below the radials the proximal plates are almost in- 

 variably very small, and in many cases so minute as to be hidden 

 by the column, thus in their insignificance affording an argument 

 against comparing them with the genitals. They seem to be early 

 developed, for they are as large in the j'oung as in the adult, and 

 do not show much increase in proportions in later geological 

 epochs. I 



In some Gyalhocrinidse, the proximal ring sometimes attains con- 

 siderable proportionate size, and it seems to have developed in 

 geological time, for we find in the Lower Silurian genus Hetero- 

 crinus that this set of plates is represented in an extremely rudi- 

 mentary stage, being only faintly visible between the sutures of 

 the basals "subradials" and these plates seem to be minute in 

 most similar Silurian genera. Yet it must not be overlooked that 

 in many cases where the proximal plates are scarcely visible ex- 

 ternally, they are quite prominent internally, often larger than the 

 " subradials." This is especially the case in forms with a concave 

 base. 



In most genera with one ring of plates below the radials, the 

 proximal plates are large and prominent, contrasting in a marked 

 degree with the proximal plates in the two forms last considered. 



There are a few genera, anomalous in structure, which cannot, 

 with our present knowledge of them, be satisfactorily brought 

 within the above comparison such as Calceocrinus, Gatillocri- 

 nus, Pisocrinus, and perhaps others. 



The Blastoids, with their uniform arrangement of plates, agree 



