PROM THE UPPER GREENSAND. 315 



and truncated vertically behind ; superior surface elevated and convex on both sides, with 

 a well-marked carina extending from the apical disc to the posterior border ; the inferior 

 surface much depressed before, and on each side of the mouth more or less elevated 

 along the region of the plastron, and presenting alternate protuberances and depressions 

 there, so that the greatest height of this species is about the posterior third of the test. 

 Ambulacral summit excentral anteriorly ; vent large, situated high up in the anal area. 



Dimensions. — Antero-posterior diameter one inch and a half; transverse diameter 

 one inch and three tenths of an inch ; height nine tenths of an inch. I have several 

 moulds of this species collected from a siliceous rock in Dorsetshire, which measured 

 more than the figured specimen, obtained from the Chloritic Marl near Chard, 

 and here drawn as our type because its shell is well preserved with many impor- 

 tant details of its structure in situ. I have two large specimens of this Urchin from 

 the Cap de la Heve. The largest measures two inches and six tenths in the antero- 

 posterior diameter, two and a half inches in the transverse diameter ; and one inch and 

 two tenths in height ; the second specimen measures two inches and four tenths in 

 length, two inches and three tenths in breadth, and one inch and four tenths in height, 

 so that when we reduce these measurements for comparison with those of the figured 

 specimen, we find the proportions are nearly the same as relates to length, breadth, and 

 height throughout. 



Description. — This is a very neat, oblong, cordiform Urchin ; the round anterior 

 border is deeply grooved by the anteal sulcus, the widest part of the body is behind the 

 anterior ambulacrum, and from that point backwards the ambitus contracts into the 

 posterior border, which is vertically truncated to form the anal area (fig. 1 e). 



The antero-lateral ambulacra curve gently up to the disc, and the postero-lateral pair 

 are nearly straight in their course (fig. 1 a). The poriferous zones consist of pairs of pores, 

 which are distant from each other below, and approximate above ; in some of the larger 

 specimens the pores are oblong below and round above. The anteal sulcus is very deep, 

 extending from the mouth to the apical disc ; it is bordered by elevations of the test, 

 which form two carinse on the sides of the sulcus (fig. 1 a, d). On these several large 

 tubercles are arranged. The single ambulacrum occupies the middle of the sulcus ; its 

 avenues consist of very small pores, which are only visible with the aid of a lens. The 

 three anterior ambulacra converge around the front of the disc, at a considerable distance 

 from the posterior pair, which converge at the hinder part thereof (fig. 1 a) . 



The test is very thin, and the inter-ambulacral areas consist of very wide plates (fig. 1 c); 

 their surface in both areas is closely covered with a very fine microscopic granulation, in 

 the midst of which a number of small tubercles, nearly equal in size, are uniformly spaced 

 out upon all the surface of the test. On the sides of the anteal sulcus, and on the cheeks 

 and plastron at the base the tubercles are much larger ; they are all raised on bosses with 

 crenulated summits, and are perforated and surrounded by a smooth circular zone, 

 defined by an outer circle of granules (fig. 1 b, d). 



