74G mi. AV. B. CAIJPEXTEK ON THE STEUCTUEE, PHYSIOLOGY, AND 



ligaments by ■which it is articulated to the Second Radial, are gradually developed into 

 the form they present in the adult ; and that the characteristic ridges and furrows of its 

 internal face (b), with the prolongations that connect it with the ventral face of the rosette, 

 make their appearance. All these features are marked out when the size of the plate is 

 still minute as compared with that which it ultimately attains ; and a little consideration 

 will show that they cannot be maintained through each subsequent stage of growth, 

 without a process of modelling analogous to that already described in the growth of the 

 centro-dorsal and basal plates, — the first-formed portions of the calcareous skeleton being 

 removed by absorption, whilst new deposits are being laid down elsewhere. If any further 

 evidence of this be needed, it will be found in the enlargement of the Canals which are 

 occupied by the radial prolongations of the centi'al sarcodic axis, the diameter of these 

 canals in the adult being at least equal to the wliole breadth of the plate in the young. 



92. Second Radiah. — The alteration which the form of the Second Eachals undergoes 

 in their progress to maturity, is even greater than that of the First ; for whilst they 

 increase but little in the direction of their original length — i. e. in the space between 

 their proximal and their distal faces, — they undergo a great augmentation both in 

 breadth and in depth (Plate XXXVI. fig. 2), their proximal face attaining an equality 

 in both dimensions with the distal face of i\\Q first radial to which it is articulated, and 

 its distal face coming to present a similarly expanded siu'face to the proximal face of the 

 third radial, in the place of the mere convexity in which the then cylindroid segment 

 terminated at an early period (§ 78). This change in the proportion of the several 

 dimensions of these plates begins to show itself very soon after the termination of Pen- 

 tacrinoid life, as is seen on comparing r", r" in Plate XLI. fig. G, and Plate XLII. fig. 7 ; 

 and, as in the preceding instance, there takes place concurrently with their increase of 

 size a gradual development of the prominences that give attachment to muscles and 

 ligaments, with a deepenmg of the cavities that lie between them, as well as a pro- 

 gressive enlargement of the central canal. 



93. Third Badials. — The change of form w'hich the Third Eadials undergo concur 

 rently with their great increase in size, is scarcely less considerable than that of ^the 

 Second; and the same tendency is manifested to lateral and vertical augmentation 

 rather than to increase in radial length. The proximal face of the plate, which is ap- 

 posed to the distal face of the second radial, rapidly increases both in w'idth and depth ; 

 and comes like it to present an expanded surface, the ventral and dorsal margins of 

 which form the bases of triangles formed by the ventral and dorsal faces respectively 

 (Plate XXXVI. fig. 3, c, d). The entire sides of these triangles now form the margins of 

 those lateral surfaces for the articulation of tlie first Brachials, which in the earlier period 

 were merely a pair of fiicets somewhat inclined to each other on the distal extremity of 

 the segment (Plate XLII. fig. 3, i^). And these lateral articular faces, as they increase in 

 proportional dimensions, come also to present prominences and fossae similar to those that 

 are characteristic of the distal faces of the First Eadials, which they nearly equal in size 

 as Avell as resemble in appearance. The central canal, with the branches into which it 



