368 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.52. 



TROPHODISCUS ALMUS, new species. 



Description. — Rays 5. R = 43 mm., r = 26 mm., R = 1 .65 r ; cotype: 

 R = 39 mm., r = 25 mm., R = 1 .56 r. General form stellato-pentagonal 

 with short, broad rays, arcuate interbrachia, and sHghtly to decidedly 

 swollen abactinal area, broadly margined by the short, mde infero- 

 marginal plates; rays evenly tapered beyond the base to a bluntly 

 pointed extremity. 



When the abactinal integument is not stretched the paxillae are 

 very compactly placed. Tliey are largest at about the middle of R 

 (at base of ray proper), whence they decrease in size only slightly 

 laterally, but gradually toward the end of the ray, where the crown 

 is onl}^ one-half to one-third the width of the largest paxillae. Inside 

 a circular area bounded by the madreporic bodj^ the paxillae rapidly 

 become smaller toward the center of the disk. The large paxillae 

 have a convex crown of 20 to 30 subtruncate, or roimd-tipped 

 slightly clavate spinelets, about 4 to 5 times as long as their breadth 

 at tip and about 2 to 2.5 times the length of the stout, sHghtly flaring 

 pedicel of the paxilla. The 10 to 15 spinelets on the convexity of the 

 tabulum are a trifle stouter than the peripheral ones. 



The abactinal plates, or bases of the paxillae, viewed from the 

 coelomic side show two principal forms. Those of the central area of 

 disk and a median band along the ray are subcircular or very broadly 

 eUiptical, the margin being shghtly crenulated or incipiently lobed in 

 most cases. The plates touch each other or even shghtly overlap. 

 On the disk they are more crowded than on the outer part of the ray. 

 On the lateral areas of rays and disk the plates are more widely spaced 

 and have 4, 5, or 6 distinct but irregular lobes. Along a narrow zone 

 next the marginal plates the abactinal plates quickly become smaller, 

 with short, overlapping lobes. 



The papulae are absent from the center of the disk and a very nar- 

 row midradial band, upon which a papula will sometimes encroach. 

 They are best developed wherever the abactinal plates have well- 

 marked lobes. 



Supermarginal plates 19, forming a broad, shghtly raised, beveled 

 margin or frame to the abactinal surface. The plates are short and 

 relatively broad, and decrease regularly in width toward the end of 

 ray. They are covered with fairly large polygonal granules, which 

 form an irregular tessellation and increase slightly in size toward the 

 outer, rounded end or edge of the plate. The granules are rather 

 squamif orm in appearance, except the peripheral which are slenderer 

 and spiniform. 



Inferomarginal plates 19, slightly broader than the superomarginals 

 and extending laterally beyond them a variable but short distance 

 (not more than a fourth the width of the superomarginals; in the type 

 less), so as to define the ambitus in the interbrachium at least. In- 



