358 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



tubercle is 2 to 2.5 mm. and its breadth at the top 2.5 to 3 mm. (in 

 truncatus similar measurements of a comparable specimen are 3.5 

 to 4.5 and 6 to 7.5). Between the tubercles are widely spaced small 

 granules similar to those around the base of the tubercles. Numer- 

 ous long, very slender, 2- jawed pedicellariae straight or curved at 

 the end stand beside many of the tubercles. Sometimes the tips of 

 the jaws are widened and truncate. Papulae small generally dis- 

 tributed. A peculiarity of the abactinal and to a less extent the 

 actinal tubercles is that they are very deciduous. The specimen 

 appears to have been well hardened, however. 



The marginal plates are difficult to differentiate from the others. 

 Eighteen inferomarginals (of which 9 underlie the large terminal 

 superomarginal) can be distinguished, although the 3 or 4 widely 

 spaced proximals are just like the abactinal tubercles. The distal 

 plates are closer together and bear each a compressed, almost fan- 

 shaped tubercle, quite different from the rest. This becomes sub- 

 conical on the distal half dozen small plates. The papulae extend 

 up to and even between the inferomarginals, so that the superomar- 

 ginals can not be positively identified. The terminal superomar- 

 ginal is larger than in any other species, broadly oval, the broad 

 end actinad, and is bordered b}^ a single row of beadlike, elliptical, 

 granules. Although the surface is curved it is not tumid. Major 

 diameter 13 mm., minor diameter, 10.5 to 11 mm. (In a large speci- 

 men of truncatus the nearly circular terminal superomarginal is 

 8 mm. in diameter.) 



Each actinal intermediate plate has a conspicuous tubercle accom- 

 panied by 1 to 3 much smaller tubercular spines of unequal size, and 

 also often a slender 2-jaAved pedicellaria such as are common on the 

 abactinal surface. The plates are surrounded by unequal pinched 

 or prismatic granules, with numerous other smaller granules in the 

 shallow depressions between the plates. The large tul)ercles are 

 globose or obovate, sometimes slightly compressed, near the margin 

 of disk, but increase in length and become decidedly compressed as 

 the furrow is approached. Those bordering the adambulacral plates 

 are strongly compressed and subspatulate. with a truncate or 

 slightly rounded, beveled distal edge. The spines of the next series 

 are not quite so long or so compressed. Usually 1 or 2 slenderer, but 

 conspicuous, spines accompany each of the large spines, while on 

 these inner plates 1 or more of the marginal granules may be consid- 

 erably enlarged. The series of plates adjacent to adambulacrals 

 reaches about as far distally as the middle of the large superomar- 

 ginal. The next series does not reach the inner end of the big 

 plate. In all there are about 7 chevrons of plates. 



Furrow spines 3, rarely 4, subequal, slender, compressed, tapering 

 slightly, round-tipped, about 1.5 times the length of base line of the 



