506 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



the supply of the two arms borne by it. The dorsal face of this segment is 

 everywhere irregularly convex, but the ventral face has a deep depression in its 

 center, the bottom of which almost reaches the axial canal. 



The alteration which the form of the IBrj undergoes in the progress to maturity 

 is even greater than that of the radial, for while tliey increase but little in the 

 direction of their original length they undergo a great augmentation both in 

 breadth and in depth, their proximal face attaining an equality in both dimensions 

 with the distal face of the radial to which they are articulated, and the distal 

 face coming to present a similarly expanded surface to the proximal face of the 

 IBrj in place of the mere convexity in which tlie then cylindrical segment termi- 

 nated at an early period. This change in the proportion of the several dimensions 

 of these plates begins to show itself very soon after the termination of pentacrinoid 

 life and, as in the case of the radials, there takes place concurrently with their 

 increase in size a gradual development of the prominences that give attachment to 

 muscles and ligaments, with a deepening of the cavities that lie between them, 

 as well as a progressive enlargement of the central canal. 



The change in form which the IBr„ undergo concurrently with their great 

 increase in size is scarcely less considerable than that of the IBr^, 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 apposed to 

 the distal face of the IBr,, rapidly increases both in width and depth, and comes 

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

 form the bases of triangles foi-med by the ventral and dorsal faces respectively. 

 The entire sides of these triangles now form the margins of those lateral surfaces 

 for the articulation of the first brachials, which in the earlier period were merely 

 a pair of facets somewhat inclined to each other on the distal extremity of the 

 segment; and these lateral articular faces, as they increase in proportional 

 dimensions, come also to present prominences and fosste similar to those that are 

 characteristic of the distal faces of the radials, which they nearly equal in size 

 as well as resemble in appearance. The central canal, with the branches into 

 which it bifurcates, is progressively enlarged by the internal resorption of its wall 

 as in the preceding cases. 



In the later stages of pentacrinoid life the elements of the IBr series exhibit 

 an increase in all their dimensions without much departure from their original 

 form. 



Each of the arms is terminated by a growing point of condensed sarcode in 

 which new brachials successively originate, a cribriform calcareous film being 

 first formed on the dorsal face and an ingrowth of fasciculated calcareous tissue 

 then taking place in such a manner as in the first instance to leave a deep groove 

 on the ventral face which is afterwards converted into a canal by the closing over 

 of its margins so that a transverse section at first resembles a horseshoe rather 

 than a ring. 



As the skeleton of the arms increases in length the vascular structures are 

 prolonged, and new groups of tentacles are developed from the extensions, each 



