ECHINODERMS. 73 



unequal, and ratber small convex scales, among which the primary onea 

 may be distinguished by their somewhat larger size ; radial shields 

 small, wide apart, more or less encroached upon on all sides by the sur- 

 rounding scales, the exposed portion being rounded or oval ; there is a 

 group of a few small scales in the notch at the base of the arms, but 

 neither the notches nor the arm-plates are bordered with papillae. 

 Lower surface of the disk covered with irregular, flattened scales. 

 Genital slits, with a series of very numerous small, grain-like papillce 

 along the outer edge, and extending around the upper end and a short 

 distance down the inner margin; on the middle of the outer margin 

 they are crowded in three or four rows, but around the outer end they 

 form a single row. Arms six, about twice as long as the diameter of 

 the disk, convex above, rapidly tapering from the base to the tip. 

 Upper arm plates, near base of the arms, short, more or less concealed 

 laterally by a group of three or four small, irregular, imbricated scales, 

 on each side, which usually do not extend across the arm, and toward 

 the middle of the arm these are reduced to a single small, triangular 

 scale, and this also soon disappears ; toward the middle of the arm the 

 upper plates are nearly as long as broad, the outer and inner edges 

 being nearly parallel and slightly curved ; farther out they become longer 

 than broad, and irregularly hexagonal, the proximal end narrowest. 

 Arm-spines three, rather short, subfusiform, tapering to a blunt point, 

 the upper one a little the^largest, the length about equal to that of the 

 upper arm-plate toward the base of the arms, but shorter toward the 

 end. Under arm-plates transversely elliptical, with a distinct angfe in 

 the middle of the proximal edge, where the lateral plates do. not quite 

 unite, leaving slight pits toward the base of the arms. Tentacle-scales, 

 beyond the middle of the arms, single, short, flattened ; from about the 

 eighth to sixteenth joints there are two tentacle-scales, the inner one 

 becoming quite small before disappearing; on the second to seventh 

 joints there are mostly three tentacle-scales, the inner oqe quite small : 

 on the first, and sometimes on the second joint, there are four tentacle- 

 scales, and also a similar group of three or four scales on the opposite 

 border of the tentacle-pores. The outer oral tentacle-pore is very large^ 

 in the form of a broad, oblique fissure, occujiying more than half the 

 length of the jaw, bordered externally by a row of about five flattened, 

 squarish scales, of which the distal one is largest, and an inner row of 

 four or five smaller ones, of which the proximal ones are situated within 



