ilONOGKAPH OF THE EXISTING CKINOIDS. 



127 





fourth of the arm gradually and slowly becoming wedge-shaped again, as a 

 result of decreasing obliquity of their ends, and slowly increasing in proportionate, 

 though not in actual, length, correlative!}' with their gradual decrease in diameter, 

 so that the terminal brachials are much longer than broad, more or less strongly 

 constricted centrally, with their ends almost or quite at right angles to the longi- 

 tudinal axis of the arm. 



The distal edges of the brachials following the first syzygial pair are more 

 or less produced and are 

 commonly bordered with 

 spines, which are usually 

 very fine, but which may be- 

 come much elongated. On 

 the proximal wedge-shaped 

 brachials this production of 

 the distal edges stands out 

 at right angles to the longi- 

 tudinal axis of the arm, and 

 if very prominent it may be 

 accompanied by a similar, 

 though lesser, production of 

 the proximal edges. As the 

 brachials become elongated 

 the latter, if present, dis- 

 appears, and the produced 

 distal edges gradually lean 

 more and more toward the 

 arm tips, so that they over- 

 lap more or less the bases of 

 the succeeding brachials (fig. 

 1140, pi. 22). W//M*' // IV I 



Usually the dorsal sur- ^UffH ///// 



face of these brachials is 

 smooth, but in certain species 

 of Thalassometridee, and 

 very rarely in other types, 

 they may carry numerous 

 fine spines. On the wedge- 

 shaped proximal brachials these spines are thickly massed in a broad band and 

 show no definite arrangement, but as the brachials elongate the broad band grad- 

 ually resolves itself into a triangle, of which the base is the dorsal portion of the 

 distal borders of the brachials and the apex lies in the mid-dorsal line near or at 

 the middle of the proximal border, while the spines themselves gradually come 

 to arrange themselves in rows parallel to the longitudinal axis of the arm. 





FIG. 198. LATERAL VIEW OF SPECIMEN OF PETASOMETEA CLAR.E. 



