1894. PROCEEDINGS OF THE NATIONAL MUSEUM. 281 



wbicli are in contact or somewhat imbricated over the greater part of 

 tlie surface and have a small conical elevation in the middle, upon 

 which there is generally 1, but sometimes 2 or 3, small, slender, very 

 acute spines of nearly uniform size over the entire surface, except at 

 the origin of the rays, where both j^lates and spines are smaller. The 

 madreporic plate is small, situated close to the margin, and has promi- 

 nent radiating gyri. 



Between the bases of all the arms and standing obliquely on the 

 margin there is a rather large, oval, interradial plate, with the surface 

 concave and bare of spines, except around the margin, wliich is more 

 or less encroached upon by small spinous disk-plates. On the central 

 pai t of each interradial plate there is a small group of pedicellariae 

 having very slender, curved jaws. A few similar pedicellarite occur 

 scattered on the disk between the spines. Opposite the base of each 

 ray, near the margin of the disk, there is a pair of snuill pores each in 

 the middle of a small naked membrane. 



The jaw-j)lates are narrow and elongated, the two together being 

 somewhat hour-glass shaped. Each jaw usually bears a pair of very 

 slender, sharp spines on the oral edge, directed inward; sometimes 

 there is also a much larger median spine in the same j)lane; on the 

 extreme inner angle on each side there is also a very small, slender 

 spine directed transversely, but the relative size and even the number 

 of these spines varies on the different jaws of the same specimen; on 

 the outer end each jaw bears a pair of much larger lateral spines which 

 stand more erect; sometimes an additional smaller spine occurs just 

 below one or both of these. On some of the jaws an additional large 

 lateral spine is occasionally found at about the middle and near the 

 margin of one plate and occasionally a pair of such spines appears. 

 All the jaw-spines are covered with groups and clusters of pedicella- 

 ria', and the larger spines are inclosed in a sacculated membrane. 



The rays ar6 very long, rather large; in the basal-genital region the 

 ray is somewhat swollen and evenly convex, but is here broader than 

 high in the dry specimen; farther out the rays gradually become slen- 

 der and angular, with a strong dorsal carina due to the ambulacral 

 plates beneath the thin membranous integument. The genital region 

 is usually prolonged and is crossed by a very large number of consid- 

 erably elevated, thin, acute, transverse ribs or carintTe, composed of coni- 

 cal and oblong elevated plates, and surmounted by a simple row of 

 numerous very slender, sharp spines, mostly arranged in comb-like 

 groups along the crest of the plates. In a well-grown specimen there are 

 on some of the rays upwards of CO transverse ridges, besides a number 

 of irregular ones at the proximal and distal portion. The ridges, how- 

 ever, are not very regular, many of them being crooked and more or 

 less interrupted, while a very few extend entirely across the ray, and 

 the number varies considerably on different rays. Where best devel- 

 oped these ribs are alternately larger and smaller; the larger ones cor- 



