46 BULLETIN 76, UNITED STATES NATIONAL MUSEUM 



(on disk upwards of eight) which increases in size on the ventrolateral region; abactinal 

 plates without miliary spinelets, which are numerous on side of ray; pedicellariae 

 fairly numerous on abactinal surface, not particularly conspicuous. Membranous 

 investment of spines, spinelets, and plates, thick and saccular in life, and extremely 

 slimy. In dried specimens it is rather insignificant. 



Description. — On the disk and proximal third of the rays the abactinal skeleton 

 is open and reticulate and unlike that of Zoroaster. The plates have usually four prom- 

 inent lobes on the rays by which they articulate with neighboring plates, leaving 

 extensive subquadrate papular areas; but on the sides and ventrolateral regions of 

 the ray the plates are more compactly placed and the papular areas are consequently 

 smaller. 



The disk of large specimens is evenly convex and higher than the rays. The prin- 

 cipal plates are a cycle of five large six and seven lobed primary radial plates, farthest 

 from center; a cycle of six-lobed primary basal or interradial plates, not quite so far. 

 These 10 plates form a pentagon of which the radials are the corners. There is a 

 good-sized stellate central plate, and connecting it with the radial plate, a smaller 

 four-lobed plate whose lateral processes imbricate with the basal plate. These five 

 plates plus the five basals make up a pentagon (usually interrupted or incomplete) 

 within the first pentagon, the basals being the corners. The papular areas of disk are 

 larger than those of the ray and are three and four sided. A circle of typically five 

 immediately surrounding the csntral plate are the largest (7 to 10 papulae) and are 

 somewhat broadly lozenge-shaped or kite-shaped. Outside of these are typically 10 

 smaller triangular meshes with five or six papulae. Outside of these a cycle of 10 

 triangular meshes (with five to eight papulae), a trifle larger than the foregoing 

 cycle of 10. 



The carinal series of plates is fairly regular; the plates are usually four-lobed, 

 rarely six-lobed, and are a trifle larger than the superomarginals, which are also four- 

 lobed. Between the carinal and superomarginal series is a very irregular series of 

 much smaller three and four lobed plates which imbricate with the processes of the 

 carinal and superomarginal plates either directly or by means of a small intermediate 

 ossicle. The characteristic large, irregular papular areas of the proximal abactinal 

 surface result from these irregular adradial plates. The superomarginal series is 

 perfectly regular and proximally the cruciform plates are a little wider than long. 

 Below these is a regular series of subequal inferomarginal plates with slightly broader 

 lobes. Then follow, at the base of ray, three actinolateral series, which are soon 

 reduced to two and finally to one on outer part of ray, these forming regular trans- 

 verse series with the marginals. The plates imbricate more and more as they near 

 the furrow (especially in small examples) so that the papular areas are reduced in 

 size, those below the inferomarginals (three series in big specimens) being smaller 

 than adjacent plates. In very large specimens a fourth series of rudimentary actino- 

 lateral plates is sometimes present for a very short distance at the very base of ray. 

 On the outer portion of ray the extreme open character of the abactinal skeleton is 

 largely lost, the adradial plates becoming more regular as the ray increases in tenuity. 

 (For details of skeleton, see pi. 15, fig. la.) 



The primary plates of the disk bear upward of six or eight short, stout, subcorneal 

 usually pointed spinules about 2 to 2.5 mm. long, and a very few smaller spinelets 



