12(1 BULLETIN 76, UNITED STATES NATIONAL MUSEUM 



specimens there is n singlt art: mil series extending less than halj the length of the ray. 

 These aetinolateral spines vary from 50 to 100 per cent longer and thicker than the 

 Buperomarginala, are quite variable in form, but are generally swollen and tapered 

 to a blunt tip; or subcylindrical; or a trifle curved; or many of the spines are com- 

 pressed, of even width; or else expanded toward the tip and subtruncate (Adakh). 



AdambulacraJ spines alternate typically one and two, fairly regularly. They 

 are much slenderer and slightly shorter than the inferomarginals, and the furrow 

 spine of diplacanthid plates is decidedly slenderer and more tapered than the others, 

 which vary considerably, form a cylindrical to a terete or compressed-clavate round- 

 tipped form. Adoral carina composed of two or three pairs of contiguous plates. 



Actinostome small; mouth plates small, with three spines each — a fairly long, 

 tapered suboral, and two apicals, of which the inner is a little longer than the median 

 oral suture; and the outer spinelet one-half or two-thirds as long; occasionally sub- 

 equal. Actinostomial margin of combined mouth plates slightly broader than the 

 opposite margin adjacent to first adambulacrals. 



Papulae in alcoholic specimens are conspicuous and appear everywhere, irregu- 

 larly, among the spines and pedicellariae. The supramarginal, intermarginal, and 

 proximally one actinal series of areas are regular. The dorsal areas are so variable 

 in size and the number of papulae per area increases so with age that it is not possible 

 to state any precise number of papulae. Anywhere from 3 to 12 occur in the dorsal 

 areas of good-sized specimens, 3 to 6 in the intermarginal, and usually only 1 to 3 

 in the actinal. 



The straight pedicellariae (pi. 48, figs. 2, 2a) afford one of the readiest means of 

 identifying specimens. Characteristic bivalved pedicellariae, having subrectangular 

 jaws a little longer than broad or slightly broader than long, of variable but large 

 size, with the edge of the jaw denticulate, occur, sometimes abundantly, on the dorso- 

 lateral region of the ray, intermarginally, and on the actinal surface, especially in 

 the interradial region. Sometimes the jaws are shaped like a miniature pecten shell. 

 These pedicellariae, when well developed, have jaws 1 mm. broad, which exceeds the 

 thickness of the abactinal spines, and even that of the inferomarginal spines, unless 

 these are exceptionally heavy. The bivalved pedicellariae are by no means uniform 

 in size. The largest are found at the base of the ray in the intermarginal and supra- 

 marginal channels, and actinally. 



Smaller ovate and lanceolate straight pedicellariae of divers sizes occur along the 

 furrow margin, on the oral spines, and in the clusters of crossed pedicellariae on the 

 furrow spines of diplacanthid adambulacral plates. 



Crossed pedicellariae (pi. 48, figs. 2c, 2d) are usually only moderately abundant 

 abactinally. They are spaced, sometimes widely, around the base of the abactinal 

 and superomarginal spines and form clusters on the outer side of the inferomarginal 

 and adambulacral spines. The abactinal measure 0.225 to 0.24 mm. long. 



Skeleton: The structural features differ from the arrangement in L. camtschatica 

 as follows: Abactinal area much broader, the superomarginals being actinolateral in 

 position; carinal series extremely irregular, sometimes not marked off from other 

 plates; dorsolateral skeleton more open, very irregular, and plates very irregular in 

 contour; marginal plates rather smaller with usually more attenuate lobes; actinal 



