



MONOGRAPH OF THE EXISTING CRINOIDS. 381 



the right, just as in Promachocrmus the pairs consist of the radial radial to the left 

 and the interradial radial to the right. 



The growth changes by which the radials reach their adult form are thus 

 described by W. B. Carpenter: "At the commencement of the unattached stage 

 the form of the radials is that of a trapezium having its upper and lower sides 

 nearly straight and parallel while its lateral margins incline toward each other from 

 above downward. Externally they still present their original cribriform structure, 

 this being particularly obvious near the upper angles where the first-formed per- 

 forated plate has not been thickened by internal addition. But while the external 

 surface is convex, being arched from side to side, the internal is nearly plane, the 

 concavity of the cribriform plate being filled up by an ingrowth of its calcareous 

 reticulation, which still retains for the most part its original type. This ingrowth, how- 

 ever, takes place in such a manner as to leave two deep channels which commence 

 from the lower angles of the plate and converge so as to meet in its center, forming 

 one large canal, which becomes completely covered in anil passes to the upper mar- 

 gin of the plate, where it opens between the articular surfaces. These converging 

 channels, when the plates are in situ, are continuous with the diverging canals of 

 the two basals, whereon each radial abuts in such a manner that the primitive canal 

 that enters each basal communicates by its bifurcation with the converging canals 

 of two different radials, while the single canal of each radial is fed by the primitive 

 canal of two different basals. At each of the lower angles of the rad-al the wide 

 embouchure of the converging canal is in proximity with that of its adjacent radial, 

 and a continuity is thus established between the several parts of this canal system 

 not only radially but peripherally. At a somewhat later period the channels are 

 completely covered hi so as to be converted into canals, and each embouchure 

 is divided by a small calcareous islet into two passages, one of them opening opposite 

 the canal of the basal, the other opposite the corresponding canal of the adjacent 

 radial. The upper margin of the radial now shows on either side of its center an 

 elevated articular surface, the calcareous reticulation of which is much closer than 

 that of the rest of the plate, and each of these gives attachment along its dorsal 

 border to a distinctly fibrous ligament connecting it with the corresponding articular 

 surface of the first primibrachs, while from the ridges which form its ventral border 

 there are now seen to pass toward the opposite face of the first primibrachs a set of 

 larger and more defined parallel fibers which, from their similarity to those occu- 

 pying a like position in the adult, we know to bo muscular. In the passage of these 

 plates from their rudimcntal to their mature condition the principal alteration that 

 we notice, besides an immense increase in size, consists in a change in the propor- 

 tions of their principal dimensions, their thickness and solidity increasing much 

 more rapidly than their superficial extension. This increase takes place in such a 

 manner that the lateral portions of the plate are brought to the same thickness with 

 the median, the dorsal and ventral surfaces becoming nearly parallel, and the lateral 

 faces come to be flattened against each other and to adhere so closely that by the 

 apposition of the five plates a solid annulus is formed. The diameter of the central 

 space of this annulus, which is occupied by the rosette, does not increase during 

 growth in nearly the same degree as that of the periphery, the size of each plate 



