30 BULLETIN 76, UNITED STATES NATIONAL MUSEUM 



ulation of one to three spinelets, these larger plates surrounded by a variable number 

 of much smaller secondary platelets which do not bear spinelets and are entirely 

 concealed by skin until the specimen is dried. In a specimen with R 12 mm. these 

 platelets are numerous everywhere among the primary plates, but in an example 

 with R 10.5 mm. the primary plates are separated by bare integument, and the sec- 

 ondary plates are beginning to appear here and there in this integument. Ordinarily 

 there are one to three delicate sharp spinelets, about 1 nun. long, to each plate, these 

 appearing blunt before being dried on account of the membranous investment. The 

 groups of spinelets are spaced 0.5 to 1.5 mm. apart and about 25 to 28 of these groups 

 can be counted from one side of the disk to the other. In some specimens half the 

 primary plates, or even more, have but a single spinelet. The spinelets as a conse- 

 quence appear to be less numerous and more widely spaced. Attached to the plates 

 and sometimes to the base of the spines are one to five minute pedicellariae. Madre- 

 poric body on edge of disk small, prominent, with a very few deep striae. 



Rays with short, swollen genital region, the length of which is about 3.5 to 4 r, 

 or on one perfect ray about one-third the length of ray. The disk plating continues 

 upon the radial region of the ray as far as the fourth adambulacral, beyond which the 

 plates are in pretty regular transverse arches, covered with an even felting of minute 

 pedicellariae. The plates are irregularly elliptical, oriented transversely, the edges 

 more or less crenulate in old specimens, and the plates are broadest on the abactinal 

 surface, narrowing conspicuously upon the sides of ray. The interval of bare skin 

 is therefore wider (1.5 mm.) on the sides of the ray, than at the crest of the arches 

 (0.5 to 1. mm.). The plates seem to increase in size with age, the arches being rela- 

 tively broadest in the largest specimens. The arches taper to a point where they 

 join the small lateral plates, and alternate arches end before reaching the corresponding 

 adambulacral, as there is no lateral plate for them to meet. The plates are thin and 

 almost flush with the general surface, except along the middle line where there is a 

 row of two to five sharp, small, spinelets articulated to an incipient ridge in old 

 specimens. The arches do not form such prominent costae or ribs as are commonly 

 found in Brisinga. Beyond the costal area the ray is crossed by fairly broad, 

 frequent bands of minute pedicellariae, one opposite each adambulacral plate. 



Lateral plates small, situated opposite alternate adambulacral plates, beginning 

 with the fifth to seventh. Lateral spines very slender and fragile, and on the outer 

 part of ray attain a length of 21 mm., or about the length of 10 adambulacral plates, 

 but on the genital region they are short. The first spine is about one plate long, and 

 the seventh about 3.5 plates (6.5 mm.). 



Adambulacral plates rather short with deeply excavated furrow margin. Arm- 

 ature consists of one aboral furrow spinelet and one subambulacral spine. The former 

 extends half-way across ray and has a very unique form. The end of the spinelet 

 flares so as to form a flattish triangluar plate, or else it is two-pronged. This tip fits 

 snugly against that of the opposite spinelet, and at the same time the tips of the 

 spines of the same series nearly meet on account of the flaring end. As a result the 

 ends of the spinelets form a series of valves which completely or nearly completely 

 fill in the spaces around the tube-feet, almost as if they had been poured in and allowed 

 to harden. This applies to the genital region, beyond which the curiously modified 

 tip becomes less prominent as one proceeds along the ray, until on the outer attenuate 



