284 BULLETIN" 100, UNITED STATES NATIONAL MUSEUM 



tubercle is rather small with the boss low; there is no trace of 

 crenulation, but there are more or less distinct radiating furrows in 

 the areole. The scrobicular ring of tubercles is rather inconspicuous, 

 the tubercles being only a little larger than the marginal ambula- 

 cral tubercles. Outside the scrobicular ring the plate is covered 

 by miliary tubercles of uniform size, leaving a rather conspicuous 

 naked sunken median line, with a fairly distinct groove at the 

 inner end of the horizontal sutures. On the adradial side of the 

 scrobicular ring there are also some miliary tubercles. The median 

 area is very narrow, only about one-third the width of an areole. 



The apical system is rather unusually large, more than half the hori- 

 zontal diameter and somewhat elevated. The ocular plates are all 

 widely exsert; the madreporite is not enlarged. The periproct is 

 rather small, with a moderate number of small periproctal plates. 

 The whole apical system is closely covered with small tubercles of 



uniform size, leaving 

 a narrow bare zone 

 along the inner edge 

 of the genital plates. 

 The genital pores are 

 large and are situated 

 close to the outer edge 

 of the plates (fig. 18). 

 The peristome is only 

 about half the size of 

 the apical system and 

 is distinctly sunken. 



Fig. 18.— Part of apical system of Psilocidakis echinulata, new There are onlv 3 Of 4 



SPECIES. X6 till- 



ambulacra! plates in a 

 series; the ambulacra join at the mouth edge. The interradial areas 

 are very small with only 1 or 2 small plates each. 



The primary spines are very long, four or five times the diameter 

 of the test; even the longest spine in the best specimen has the point 

 broken; in the second specimen not one of the ambital spines is 

 complete. These spines are straight, very slender, cylindrical, taper- 

 ing very gently to the (apparently) fine point. They are covered 

 with small spinules which are erect or only very slightly curved dis- 

 tally; these are only very indistinctly arranged in about 8 longitudinal 

 series; they rise directly from the surface of the spine, not from distinct 

 ridges, and remain of the same size throughout the entire length of 

 the spine. The surface of the spine is otherwise covered by very 

 small, simple conical " hairs." There is no indication of a basal disk. 

 The collar is about 1 mm. long, thickening toward the very incon- 

 spicuous milled ring. The apical spines are of full length, and there 

 is no indication of apical disks. The oral primaries are very slender, 



