A MONOGRAPH OF THE EXISTING CRESTOIDS 127 



The 10 arms are probably about 80 mm. long. The proximal brachials are triangular 

 or quadrate, rather longer than broad, and the distal are laterally compressed and over- 

 lapping so as to become carinate. The figures show the brachials in the outer two- 

 thirds of the arms with long, carinate, and curved median overlapping spines. The 

 inner faces of the second brachials and of the hypozygals of the first syzygial pair are 

 slightly flattened. 



Syzygies occur between brachials 3+4, again between the thirteenth and twenty- 

 sixth brachials (usually about the sixteenth), and distally at intervals of from 2 to 16 

 (usually 4 or 5) muscular articulations. 



Pi, which is much larger than P 2 , consists of about 12 segments of which the first 

 six are wide and thick with their outer sides somewhat flattened, and the third-fifth 

 have their inner edges produced into expanded processes which are slightly folded up- 

 ward. The next 3 or 4 pinnules on either side are quite small, and the length gradually 

 increases, the later pinnules becoming styliform with elongated segments. On some 

 arms the first two segments of the distal pinnules are rather expanded and trapezoidal, 

 though on others they are not specially modified. 



The disk is 6 mm. in diameter. The disk and the brachial ambulacra are well plated. 

 The side- and covering-plates along the pinnule ambulacra are generally well differen- 

 tiated. Sacculi are largely developed on some of the pinnules, though altogether absent 

 on others. 



The color in alcohol is light brownish white. 



Notes. — The preceding description is adapted from Carpenter with additional 

 information derived from his figures. 



Carpenter said that in this species the IBr t are relatively longer than those of S. 

 lusitanica and more distinctly incised by the axillaries, which are hexagonal rather 

 than pentagonal as in that species, while in the younger individuals both elements of 

 the IBr series show distinct indications of a median ridge like that which is so marked 

 in S. spinicirra and S. acutiradia. He said that the characters of the cirri and of Pi 

 also separate S. breviradia from S. lusitanica, "which was probably without such dis- 

 tinctly carinate outer arm-joints as occur in Antedon [Stiremetra] breviradia." He 

 said that some of the later pinnules have the lower segments flattened and expanded 

 as in Aglaometra valida, while in others there is but little trace of this peculiarity. There 

 is a similar variation as regards the sacculi. On some pinnules they are abundant, 

 alternating regularly with the side plates; on others there are very few; and some pin- 

 nules are altogether without them. 



Carpenter remarked that the characters of the calyx undergo a considerable 

 amount of change dining development. The centrodorsal is deeper and more conical 

 in the older individuals, in which the more numerous cirrus sockets are arranged in 

 tolerably regular columns. There are two of these columns under each interradial angle 

 of the calyx, each with 3 sockets which alternate with those of adjacent columns, and 

 the dorsal pole is covered with a number of short stout processes of which there is but 

 little trace in a younger individual. The two also differ in the characters of the radial 

 pentagon. In the younger one its lower face is tolerably flat and smooth, with the 

 rosette rather near the surface and little or no indication of a basal star, while in the 

 older it is more deeply sunken within the axial opening and is surrounded by a fairly 

 definite basal star. The surface of each radial is also very convex and rises to one or 



