288 TWENTIETH REPORT ON THE STATE CABINET. 



surface of rays composed of three ranges of large, highly convex or 

 tuberculiform plates which are nearly circular at the bases of the rays, 

 becoming quadrate and widened towards the extremities ; separated from 

 each other in the lower part by numerous minute plates or granules, 

 which become fewer near the middle of the ray, and disappear before 

 reaching the extremity. 



The central portion of the disc is occupied by an elevated pentagon, the 

 angles of which are formed by the abrupt termination of the central row 

 of plates of each ray : the whole composed of very minute, highly con- 

 vex plates, which vary in size, the larger one pentagonally arranged. 

 The angles between the rays have a few small plates outside of the outer 

 ranges of tuberculose plates on the upper side, uniting with the marginal 

 plates below. Madreporiform tubercle distinct, situated laterally at the 

 bases of the outer range of large plates of two adjacent rays. 



Ventral surface having deep ambulacral grooves, bordered by two ranges 

 of strongly tuberculose plates ; the outer marginal range consisting of 

 twenty-seven or twenty-eight plates, besides a large, round, terminal 

 or axillary plate ; the others are wider than long in the basal portion of 

 the ray, becoming gradually shorter towards the extremity where they 

 are rounded. 



All the marginal plates are visible from the upper side, and usually appear 

 as an additional range of plates on each margin of the ray, making five 

 with the three properly belonging to the upper surface. 



The inner range bordering the ambulacra (adambulacral plates) are smaller 

 than the marginal plates, about thirty-eight to forty in number ; the basal 

 or oral plates are triangular, those of the adjacent rays uniting by their 

 longer margins ; and with a single minute plate situated at these points. 



The plates of the exterior surface, both upper and lower, present a granu- 

 lose or striato-granulose surface which appears to have been produced by 

 short setae or spine ; and at the angles of the rays the marginal plates 

 are armed by a few spines, which are as long or longer than the trans- 

 verse diameter of the plates. 



Ambulacra composed of a double range of short broad poral plates 

 (ossicula), equal in number to the adambulacral plates; their outer ends 

 excavated on the posterior border, forming a comparatively large pore, 

 just within its junction with the adambulacral plate. There appears to 

 have been but one range of pores in each set of ossicula, but these are 

 large, distinct, and pass between the plates. 

 In the collection there is an impression of a single ambulacral area of 



this species, which is spread open laterally and measures about two and a 



half inches in length by nearly three-fourths of an inch in width in the 



middle, broadly petaloid in shape, and showing the form and number of 



poral plates, with the position of the pores and their junction with the 



adambulacral plates. 



8 



