280 THE FOSSIL ECHINOIDEA 



cannot in consequence be given accurately ; but a smaller example (in which the height 

 would normally be greater) presents the above proportions exactly. When seen in 

 profile, the inflation at the ambitus presents a very regular curve, full and well rounded, 

 the contraction of the test being very little, if at all, greater actinally than abactinally. 



Ambulacra considerably flexed, rather narrow, contracting towards the apex, and 

 to a less degree towards the peristome. Poriferous zones rather shallow and not very 

 deeply sunken, nearly as broad as the interporiferous area. Pores slightly oval trans- 

 versely, those of a pair separated from one another by a space rather greater than their 

 own transverse diameter, upon which is present a rather coarse and ill-defined elevation, 

 limited to the aboral portion of the plate, the adoral margin of the plate having the 

 appearance of being faintly channelled, but not definitely in connection with the pores, 

 the feature being mainly produced by the thin elevated ridge developed on the aboral 

 margin of the adjoining plate, by which the pairs of pores are separated. Near the apex 

 the ill-defined elevation which separates the pores of a pair becomes more definitely 

 granular. The two pores on a plate are similar in shape and size : and there are 16 or 

 1 7 pairs of pores opposite to one of the largest interambulacral plates at the ambitus. 

 The interporiferous areas are of moderate breadth and are furnished with two rows of 

 minute mammillated tubercles, placed at the extreme margins of the area, that is to say 

 one on each plate adjacent to the inner row of pores. On the abactinal portion of the 

 area the plates bear on the inner side of this small tubercle one still smaller miliary 

 granule, in regular series ; in the neighbourhood of the ambitus, however, the place of 

 this granule is occupied by two miliary granules, standing one above the other, so that 

 the regularity of the uniserial row of the inner miliary granules is unbroken. 



The interambulacral areas are rather more than five times the width of the ambu- 

 lacra at their widest part : and there are two rows of 7 to 8 primary tubercles in each 

 interambulacrum, the tubercle, however, on the uppermost pair of plates being usually 

 undeveloped. The primary tubercles are large, perforate, and non-crenulate, and their 

 bosses are large and extensive, frequently somewhat tumid. The mamelons are broad 

 and subdepressed, somewhat button-shaped, with the edge slightly bevelled or flattened 

 in its rounding. The collar is wide and sensibly grooved, the neck is short and thick. 

 The scrobicules are wide and extensive, circular on the abactinal surface, but becomino- 

 oval at the ambitus, and in an increasing degree towards the peristome. The scro- 

 bicular ring is complete, and composed of 15 to 16 conspicuous mammillated granules, 

 or miliary tubercles, widely spaced. The mamelon of these granules is small, spherical, 

 and well defined ; the boss broad, subdepressed, and more or less expanded laterally, 

 and often somewhat undulating in its surface. The scrobicular ring comes so close to 

 the aboral and adoral margins, and the margin adjacent to the poriferous zone, that 

 there is only room for single very minute miliaries here and there opposite the inter- 

 spaces of the miliary granules of the ring. In the case of the margin adjacent to the 

 poriferous zone the miliaries are rather larger and more frequent, but only a single row 

 is present, except at the upper and lower corners of the plate, which are filled in with 

 two or three additional ones. The miliary zone is very narrow, with a well-defined 

 depression along the zigzag median suture ; and more emphasized depressions occur at 



