22 BULLETIN 76, UNITED STATES NATIONAL MUSEUM 



numerous along the mid-dorsal line; disk with solitary spinelets spaced about their 

 own length; first 12 or 15 subambulacral spines with a truncate, circular flaring tip; 

 proximal plates with three aboral spinelets in a transverse series, and a subambulacral, 

 also close to distal border; plates of outer part of genital region with two aboral 



spinelets ( " + 1 or J; no transverse rows of more prominent abactinal 



spines; lateral spine opposite alternate adambulacrals, longer than the subambulacral 

 spine. 



Description. — The rays are very long, narrow, tapering gradually, a trifle nar- 

 rower at the base than throughout the very extensive genital region, which is not 

 inflated at all conspicuously. The rays are subcylindrical at the base, depressed on 

 the genital region and of the ordinaiy three-sided form beyond the genital region. 

 The latter, or, more correctly speaking, the portion of the ray covered by the small 

 papery abactinal plates, comprises nearly or sometimes quite half the total length of 

 ray. 



The edge of the fairly large disk is evenly rounded and somewhat undulating 

 in outline. The abactinal integument is very tight and is beset with equal, solitary, 

 small, terete, blunt spinelets, spaced about their own length, those about the anal 

 aperture being a trifle longer than the others. The shape of the spinelets is due to 

 the membrane investing each one, as the calcareous portion is acicular. Interradial 

 plate rather high and narrow, confined to side wall of disk. 



The abactinal membrane of rays is rather thin and papery, the delicate plates 

 being invisible except in a thoroughly dried specimen. They are very thin, consist- 

 ing of a single layer of delicate calcareous network, and are irregular in outline. The 

 plates are not uniform in size and overlap by the edges. Over the extensive genital 

 region the integument is beset with minute, evenly spaced prickles or spinelets, 

 incased in membrane which gives them a blunt appearance. There are one to three , 

 commonly one or two, to each plate, and they are shorter and more widely spaced 

 than on the disk; those of the midradial area are more closely placed than are tbose 

 on the sides of the ray. On the outer part of the genital region the prickles are still 

 more widely spaced and many of the plates are without them. Except on the first 

 50 mm. of the ray, the indistinct bands of microscopic pedicellariae from each lateral 

 and ambulacral plate proceed toward the median radial line, before reaching which 

 they break up. On the semitransparent membrane of the outer half of the ray, 

 where there are no integumentary prickles, these bands of pedicellariae are more 

 conspicuous, and extend entirely across the abactinal surface, but are frequently 

 irregular on the median keel of the ray. Extending upward from the rudimentary 

 marginal or lateral plates are a few plates stouter than the other abactinal plates 

 which sometimes reach halfway to the median radial line. 



Partly fused to the lateral face of alternate adambulacral plates is a lateral plate, 

 bearing a long, slender spine (13 mm.) sheathed in membrane covered with micro- 

 scopic pedicellariae. (On plates adjacent to the lateral spine, the subambulacral 

 spine is conspicuously shorter than on alternate plates.) At the base of ray are 

 five marginal plates without spines. The first of these is strongly fused with its 

 fellow of the adjacent ray, at the apex of the interbrachial angle and above the first 

 adambulacral plates. 



