DEVELOPMENT OF ANTEDON (COMATULA, LAMK.) EOSACEUS. 745 



line of each plate, until the external layer has been completely removed from its salient 

 angle (fig. 3, h), tlie two lateral portions of that layer are separated from each other ; 

 and remain only as a pair of curved processes extending themselves from the inner 

 layer in such a manner as to give to the plate when viewed from its ventral side some- 

 what the aspect of a saddle (fig. 3, c, d). AVhen the five Basals thus altered are in 

 their normal apposition, the curved process on either side each plate comes into contact 

 with the coi-responding process of its next neighbour; and the junction of the two 

 forms a sort of ray curving towards the dorsal aspect. As each plate thus contributes 

 the half of two of these curved rays, five such rays are formed between the five salient 

 processes which are put forth by the internal or ventral layer, on the median lines of 

 the five plates, and are received into the retreating angles formed by the junction of the 

 First Eadials. Very soon an actual continuity is established in the calcareous reticula- 

 tion along the lines of junction, and the " rosette" is completed, as shown in Plate XLII. 

 fig. 1, although the peculiarity of its shape becomes much more strongly pronounced 

 with the subsequent increase of its size (Plate XXXIII. figs. 9-11). — Thus we see that 

 the "Eosette" is essentially formed at the expense of the secondary or ventral layer 

 of the original Basals, the ends of the curved rays being the sole residue of their primary 

 or dorsal layer ; and since, by the removal of the median portion of that layer in each 

 plate, the radial cords are left bare on their dorsal aspect, they now pass from the cen- 

 tral axis into the canals of the First Eadials on the outside of the calcareous skeleton 

 which occupies the central part of the base of the Calyx, instead of reaching these by 

 passing (as they did in the first instance) along its internal face, or (as at a later period) 

 through the middle of its substance. 



91. First Badials. — In the passage of these plates from their rudimental to their 

 mature condition, the principal alteration that we notice, besides an immense increase in 

 size, consists in a change in the proportions 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 its periphery, the size of each plate 

 (it would seem) being more augmented by addition to its external face than to its lateral 

 faces ; so that the ratio of its breadth at its inner and its outer Inargins, instead of being 

 (as at the conclusion of Pentacrinoid life) about 2 : 3, comes to be only 1 : 3, the shape 

 of its dorsal face being thus changed from a trapezoid 

 to a triangle with its apex truncated. Concurrently with 

 these changes, we find that the various ridges and fossae 

 on the external and ventral faces of the plate (§ 33, Plate 

 XXXVI. fig. 1, A, c), for the attachment of the muscles and 



5i2 



