100 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



three short, thick, blunt pointed spines. The [infra] marginal plates 

 bordering each reentrant angle bear similar but more slender spines, 

 which are not 'as long as the transverse diameter of the plates.' 

 The spines are arranged in a row near the distal margin of the plates 

 and number five on the plates at the angle, the number and size 

 decreasing until they disappear at the sixth or seventh plate from 

 the angle. All the [infra] marginal plates are nearly smooth on the 

 free margin and become gradually more granulose toward the line 

 of junction with the adambulacral plates. * * * 



"The adambulacral plates are apparently less numerous than 

 stated in the original description, and l the single minute plate 

 [mouth plates] ' at the points of the pairs of the oral plates [oral 

 armature] is visible in this specimen and is armed with two relatively 

 long, slender spines which are apparently but a part of the full 

 armature. The adambulacral plates, including the triangular oral 

 [armature] plates bear well-defined spines, which are shorter than 

 the diameter of the plates to which they are attached. Each plate 

 bears two spines so near to the distal margin that the impressions 

 of the short and obtusely pointed spines frequently bridge the 

 well-defined groove between the adjacent adambulacral plates and 

 terminate near the proximal margin of the next plate. The spines 

 decrease in size toward the end of the ray and a few plates show 

 only one spine. The plates of this range are thick, equaling two- 

 thirds to three-fourths the depth of the groove. The vertical angle 

 of the faces forming the lateral walls of the groove are beveled, so 

 that lateral extensions of the groove are formed between each two 

 plates on the same side. These lateral expansions are narrow and 

 shallow at the oral surface, deeper and wider inward; so that the 

 faces of the adambulacral plates near their junction with the poral 

 [ambulacral] plates are reduced to a narrow edge which projects 

 inward and nearly touches the corresponding plate on the other 

 side of the groove. The general appearance of the fossil as well as 

 the outline of the rays at the points where the broken block presents 

 a transverse section of them indicates that the plates have their 

 normal position, not having suffered distortion by pressure. 



"The ambulacral plates are shown by a well-defined mold of their 

 under or external surface. The soft matrix which filled the ambula- 

 cral furrow pressed upon the membranes connecting the ambulacral 

 plates and occupying their pores, and as these membranes decayed 

 it was forced by gentle pressure into the pores and between the 

 edges of the plates. The mold of the groove is less than one-eighth 

 of an inch in width in a ray measuring five-eighths at its base. The 

 upper surface of the mold bears a narrow longitudinal median ridge 

 which marks the junction of the two ranges of ambulacral plates. 

 Similar transverse ridges, which are continuous with the lines mark- 



