278 BULLETIN 76, UNITED STATES NATIONAL MUSEUM. 



mediate plates. The spinulation is so dense all along the adambulacral plates 

 that it is dillicult to distinguish the individual plates. 



Mouth plates comparative^ large, rather prominent actinally, this appear- 

 ance bein" accentuated by a depression in the interradial area at their outer end. 

 General surface is covered with numerous, robust, prismatic, subquadrate, trun- 

 cate, short irregular spinules or spines, which increase in size and become more 

 compressed (broad side to furrow) toward the free margin of plate. The true mar- 

 ginal series begins near outer (aboral) end, high in the furrow, next to first ambulacral 

 plate as rather inconspicuous flattened lanceolate or truncate spinelets, wliich 

 rapidly increase in size toward inner angle. These form a group or an angular series 

 (the apex being next to peristome), one part of which curves upward (as viewed 

 from actinal surface) along the fmTow margin of plate, while the other, closely 

 parallel for a way, proceeds upward to the inner or dental angle of the plate. At 

 tliis inner or dental angle is a large median spine du-ected over actinostome, as 

 in PseudarclMsttr. This spine is either flattened and broad at tip (type), flattened 

 without being conspicuously widened (station 4784), or truncate and somewhat 

 pestle-shaped (station 2879). On either side of the median tooth is a similar spine, 

 and those on actinal surface seem to grade into these dental spines. The median 

 spine is not always present. On the outer part of the plate of the largest specimen 

 the small irregular but very robust spinelets have a truncate concave tip. There 

 is so much variation in the minor details of the armature in the three specimens 

 that it seems useless to describe in detail the actinal surface of these plates. The 

 illustration (pi. 57, fig. 2) \vill give a better idea. 



Actinal interradial areas large and paved with superficially elliptical or oblong 

 plates arranged in series from aclambulacrals to inferomarginals. There is one of 

 these series to each adambulacral (as far as the plates extend) with occasionally an 

 extra series. The plates of a series imbricate, the outer end underlying the inner 

 end of the next exterior plate, but although the independent series are close together, 

 the plates do not appear to overlap along the longitudinal axis of ray. On the 

 rays the intermediate areas are narrow, not broad as in M. tizardi. One series 

 of plates Gongitudinal) extends nearly to tip of ray; the second series extends 

 only half tlie length of fay measured from first marginal plate; a third series about 

 one-fourth or to eleventh inferomarginal. Between this point and interradial 

 line the numVjer of plates increases very rapidlj'. Each plate is slightly convex 

 and is covered with coarse spinelets, wliich are largest in center of plate, clavate, 

 prismatic, quadrate, or pinched, with subtruncate or rounded tips. The peripheral 

 spinelets are smaller truncate, jnnched or papilliform, and bend over the shallow 

 channels between the transverse series of plates. These are similar to the "post- 

 adambulacral fascioles" of Pseudar chaster, but the grooves lead all the way to the 

 marginals. A few of the intermediate plates on largest specimen have fom- central 

 spinelets equal and closely grouped to resemble a pedicellaria; I doubt its being one. 



Madreporic body inconspicuous about midway between center of disk and 

 inner edge of marginal plates or a little nearer center than margin. It is small 

 circular convex, partly overhung by paxilhe, a large primary basal paxilla occurring 

 on the adcentral side. Striations fine, ridges coarse centrifugal, branching and more 

 or less sinuous. 



