450 PROCEEDINGS OF THE NATIONAL MUSEUM vol.90 



larly oblong. On the disk the skeletal intervals are very small — 

 about the size of the primary lobed plates — but on the rays they are 

 more open, especially above the inferomarginals where they are very 

 large. The inferomarginals, which are applied to the outer end of 

 the adambulacral plates, are 3-lobed and not directly imbricated: 

 There is a secondary elliptical ossicle between any two. The infero- 

 marginals proximally often have two acicular spines the length of 

 three adambulacral plates but usually there is one. Applied to 

 upper lobe of inferomarginals is an oblong plate, not different from 

 others above it, which probably represents the superomarginal. But 

 there is no clearly differentiated series of superomarginals or of carinal 

 plates. The latter can not be distinguished from among the numerous 

 abactinal plates. 



Most of the abactinal plates carry a slender, sharp spinelet 1 to 

 1.5 mm. long, surrounded at the base by four or five crossed pedicel- 

 lariae, 0.35 to 0.45 mm. long, having one prominent tooth on each 

 side of the tip of jaw and very numerous shank teeth in several longi- 

 series. In the aggregate these pedicellariae are very numerous, but 

 thin out on the sides of ray, proximally. The proximal inferomargi- 

 nal spinelets usually carry basally only one or two small lanceolate 

 straight pedicellariae but beyond middle of ray are equipped with 

 a circlet of crossed pedicellariae. The straight pedicellariae are also 

 scattered over the surface of the body, usually on papular areas, and 

 are one-half to three-fourths the length of spinelets. Papulae numer- 

 ous but difficult to determine accurately because the specimens 

 have been dried. On the disk each small mesh has one papula. 



The adambulacral plates are short, rather broad, and characteris- 

 tically curved. They carry three slender spines, obviously longer 

 than the inferomarginals and without any attached pedicellariae. 

 However, on the furrow face of the plate, near base of the furrow spine, 

 one or more pedunculate straight pedicellariae of different sizes 

 usually occur. The adambulacral and oral spines are the longest 

 of the body and form a dense cheval-de-frise on either furrow margin. 

 The small mouth plates each carry one prominent suboral and two 

 often flattened actinostomial spines, as well as a variable number of 

 pedunculate lanceolate straight pedicellariae. The adoral carina is 

 composed of the first seven pairs of adambulacral plates; the first 

 five may carry three, two, or occasionally only one spine. 



In Labidiaster the mouth plates of good-sized specimens become 

 so crowded that the inner or actinostomial angle meets and fuses 

 with that of the opposite side of furrow in such a manner that the 

 radial nerve has to pass thi'ough a hole arched over by these fused 

 plates. In Plazaster there is no fusion of the mouth plates. For one 

 thing the actinostomial border of the plates is not so expanded as in 

 Labidiaster, 



