MOKOGRAPH OF THE EXISTING CKINOIDS. 381 



the right, just as in Promachocrinus tho pairs consist of the radial radial to the loft 

 and the interradial radial to the right. 



The growth changes by which the radials reach then- adult form arc thus 

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

 the form of tho radials is that of a trapezium having its uj)per and lower sideS 

 nearly straight and parallel while its lateral marguis inchne toward each other from 

 above downward. Externallj^ they stiU present their original cribriform structure, 

 this being particularly obvious near the upper angles where the lirst-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 ])lane, the 

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

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

 ever, takes place in such a manner as to leave two deep chaimels which comm(>nce 

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

 one large canal, which becomes completely covered in and 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 diverguig 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 jjrimitive 

 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 in so as to be converted mto canals, and each embouchure 

 is divided by a small calcareous islet into two passages, one of them openuig 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 ol 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 corrcsjionding 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 ])rimibrachs a set of 

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

 pying a Mke position in the adult, we know to be muscular. In the i)assage of these 

 plates from their rudimental to their mature condition the ])rincipal alteration that 

 we notice, besides an immense increase in size, consists in a change iii the i)ropor- 

 tions of their principal dimensions, their thickness and sohdity increasmg much 

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

 manner that the lateral portions of the plate are brought to the same tliickness 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 sohtl amiulus 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 jilate 



