FROM THE CORNBRASH. 281 



The ambulacral areas are wide, convex, and prominent, and form an exception to 

 the generalisation of M. Agassiz, that in the genus Pygaster the ambulacra are furnished 

 with only four rows of tubercles, for in this species at the widest part of the area there 

 are six rows of well-developed tubercles (fig. I a, b) ; the two marginal rows, with about 

 forty-five tubercles in each row, extend from the peristome to the disc ; the second rows 

 commence at a short distance from the mouth, and extend two thirds of the length of 

 the upper surface ; the third rows commence at the base, about half an inch from the 

 border, and extend to about the same distance up the sides of the area (fig. 1 b). 



The poriferous zones are straight, the pores have moderately thin septa, with only 

 slight elevations on the surface, and there are four pairs of pores opposite each inter- 

 ambulacral plate (fig. 1 d). 



The inter-ambulacral areas are three times the width of the ambulacral; at the 

 circumference they are furnished with twenty-two rows of tubercles, which attain a 

 greater size and are arranged with more regularity than in any other English species ; 

 each plate above the circumference has from eight to nine tubercles developed on its 

 surface (fig. 1 d) ; those on the half of the plate nearest the centro-suture are arranged 

 horizontally on the same line, whilst the tubercles nearest the zones form double rows in 

 oblique pairs. The tubercles are raised on prominent bosses, and surrounded by wide, 

 sharply defined, sunken areolas (fig. 1 d) ; each areola is surrounded by a circle of granules 

 (fig. 1 e) ; there are thirty-six pairs of plates in each area, twenty-one of which are dorsal 

 and fifteen are basal. The tubercles attain their greatest development at the base, where 

 they are so uniformly arranged, and so closely set together, that they appear to arise from 

 hexagonal spaces (fig. 1 b) ; the areolas are deeply excavated (fig. 1 /), and have a square 

 or hexagonal figure, two sides being bounded by granules, the other two by sharp 

 elevations of the areolar border (fig. 1/); of the twenty-two rows of tubercles which 

 occupy the area at the circumference, only six, and these the three central rows of each 

 column, extend from the peristome to the disc, the others disappear at shorter distances, 

 the length of the rows being in proportion to their distance from the margin of the columns. 



Although the upper surface of this urchin is much depressed, its sides are tumid, as is 

 well seen in the profile (fig. 1 c). The single inter-ambulacrum is much truncated 

 (fig. 1 a), and the large anal opening, which has an oblong shape, occupies the upper 

 three fourths of this area ; the discal opening is small, and the plates are all absent. 



The marginal angle is obtuse, the outer half of the base is convex, and the inner half 

 concave ; the small mouth opening, which is about one sixth the diameter of the test, lies 

 in a deep depression. 



The spines are short, stout, needle-shaped bodies, marked with fine longitudinal 

 lines. 



Affinities and differences. — This species resembles P. laganoides, Agassiz, in its 

 depressed form, obtuse basal angle, and truncated single inter-ambulacrum; but 



