FROM THE UPPER GREENSAND. 291 



varies in different specimens, and the posterior is always higher than the anterior half of 

 the test. The sides are much inflated, and the base is slightly convex (figs. 2 a and 2 b). 



The anterior single ambulacrum is a third wider than the pairs. According to Forbes 

 it preserves nearly a uniform width, and is composed of plates which become contracted 

 vertically throughout the upper half (fig. 2 a). Out of the thirty-seven plates seen in each 

 of its two series the lower seven are square or oblong and large (fig. 2/). They are each 

 perforated near their ohter and lower corners by a pair of minute approximated pores. 

 In the narrow plates above these a very curious change takes place in the avenue arrange- 

 ments for the pairs of pores ; all become conspicuous, are alternately approximated, and set 

 widely apart, the latter pairs each consisting of a long outer pore and a shorter inner one 

 (fig. 2/). Inside of the pores there is a very small tubercle or two on each plate, the rest 

 of the surface being covered by minute and closely set granulations ranged in transverse 

 rows. The antero-lateral ambulacra are undepressed and superficial, and exhibit a slight 

 and graceful curve ; they are composed of an inner series of minute and approximated 

 pairs of pores, and an outer or hinder series in which the pores of each pair are widely 

 separated and unequal (the outer ones being the longest), and connected by a shallow 

 groove. There are about thirty pairs of pores in the petaloidal portion of each series ; 

 the petals are also plain and undepressed, but are more regularly lanceolate in shape in 

 consequence of the two series of pairs of pores in each being of nearly similar structure 

 and but slightly unequal in width. There are about eighteen pairs in each petaloid avenue. 

 A few tubercles (fig. 3 h) are seen upon the lateral ambulacra. On the interambulacral 

 spaces they are much more numerous though set well apart and much larger. They are 

 largest on the anterior segments, where they have wide areolae (fig. 3 c). Their inter- 

 stices over the whole of the test are occupied by minute granules, which are, however, not 

 so small or so regularly arranged as those on the odd ambulacral plates. On the under 

 surface the tubercles are confined for the most part to the interambulacral spaces, and 

 are especially numerous, regular, and closely set on the ovato-lanceolate post-oral space 

 formed by the inferior portion of the hinder ambulacrum (fig. 2 h). This space exhibits a 

 prominent caudal gibbosity. The tubercles are minute and perforated ; they are elevated 

 upon crenulated bosses (fig. 3 f/). The tracks of the ambulacra are naked. The spines 

 are unknown. 



The apical disc is small, and is composed of four perforated genital plates, the 

 perforations of which are approximated. In the midst of tliem is seen the madre- 

 poriform body. A fifth genital plate is imperforate; the five ocular plates are all 

 perforated, but very minute (fig. 2/). 



Affinities and Differences. — It very much resembles E. Filtoni in its form, outline, 

 and tumidity. It is, however, a larger Urchin, with larger tubercles on the upper surface, 

 and having them more sparsely distributed on the plates. 



Locality and Stratigrapltical Position. — The specimens I have figured are in 

 the Museum of the Royal School of Mines, and are the same which were so beautifully 



