REPORT ON THE ECHINOIDEA MORTENSEN 297 



of the plate remaining naked except for a very small miliary tubercle; 

 the interporiferous area thus is rather naked; it may be somewhat 

 sunken toward the middle line. The pore zone is scarcely at all 

 sunken; the pores are equal in size, and are separated by a fairly 

 broad and slightly raised wall; the ridge separating the adjoining pore 

 pairs is low and rounded (pi. 75, fig. 3). 



In the interambulacra the areoles are very large and flat, scarcely 

 at all deepened, showing a rather unusually distinct radial striation, 

 but without a trace of crenulation; the boss is very low, the mamelon 

 of ordinary size. At most the two proximal areoles are confluent, 

 and only these proximal ones are more or less transverse oval. In 

 the fourth or fifth the upper edge of the areole is generally straight, 

 whereas the rest of it remains circular. The scrobicular ring is 

 rather conspicuous, the tubercles being fairly large, three or four times 

 as large as the marginal ambulacral tubercles. Outside the scrobicu- 

 lar ring there are some few miliary tubercles, leaving a narrow naked 

 median space and also a narrow naked space on the adradial edge. 

 The median area is very narrow, only about one-third the width of 

 an areole. 



The apical system is about 41 to 50 per cent of the horizontal diam- 

 eter, relatively more in the smaller specimens than in the larger. 

 The oculars are all rather widely exsert; only in one specimen ocular 

 I is very narrowly insert. The oculars have the shape of a more or 

 less acute triangle, with the inner sides nearly straight. The genital 

 plates are rather high; the madreporite is not enlarged; the genital 

 pore is near the edge; the female genital pores are small. The peri- 

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

 The whole apical system is more or less thickly covered with small 

 fairly uniformly sized tubercles, some of which are more or less 

 comma-shaped (fig. 21). 



The peristome is conspicuously smaller than the apical system, 

 about 28 to 35 per cent of the horizontal diameter, and like the apical 

 system relatively larger in the smaller specimens. There are 9 or 10 

 ambulacral plates in a series in the largest specimen, but only from 

 6 to 8 in the specimen 27 mm. in horizontal diameter. In the larg- 

 est specimen the pore series is slightly irregular, some of the pores 

 being pushed slightly aside for want of space. The ambulacra do 

 not join at the mouth edge, leaving a free passage for the interradial 

 plates, which are from 3 to 6 in number and more or less irregular. 



The primary spines are about two and one-half to three times as 

 long as the diameter of the test, slender, 2 to 3 mm. thick, cylindrical, 

 tapering very gradually to a rather fine point; the longer ambital 

 spines are rather distinctly curved in the basal part. They are set 

 with about 12 to 15 longitudinal series of low, rounded spinules which 

 are generally not united at their base, thus not forming longitudinal 



