FROM THE UPPER GREENSAND. 311 



inter-ambulacrum (occasionally only) more or less elevated into a carina ; under surface 

 flat ; peristome near the anterior border. Periprocte oval, situated in the upper part of 

 a small truncated posterior border. Ambulacral summit subcentral, a little nearer the 

 anterior border. 



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

 one inch and four tenths ; height eight tenths of an inch. These are the dimensions 

 of the average number of adult specimens obtained from the Upper Greensand of 

 Wiltshire. 



Description. — The body of this Urchin has a very regular figure, of a cordato-ovate 

 shape, larger before and tapering behind, the largest transverse diameter being across 

 the middle of the postero-lateral inter- ambulacra. The anteal sulcus is very shallow, 

 and the anterior border only slightly depressed; the posterior border is narrow and 

 obliquely truncated a little downwards and inwards. 



The upper surface is in general elevated and convex (PI. LXXII, fig. 1 b, c, d, e) t 

 and some specimens have the posterior half of the test a little more ridged, and the 

 middle line forming a blunt carina, which has been erroneously considered a specific 

 character by several authors (fig. 1 c, d). 



The ambulacral areas are wide, apetaloid, and disjoined at the summit ; the antero- 

 lateral pair are arched backwards and forwards in their upper thirds (fig, 1 b), and the 

 postero-lateral pairs slightly bent in the opposite direction (fig. 1 b). The single ambulacrum 

 (fig. 1 b, e) is lodged in a wide, shallow anteal sulcus, the depression from which vanishes 

 in the upper half of its length (fig. 1 e), so that the convexity of the dorsal surface is 

 almost complete (fig. 1 b). The poriferous zones are narrow, the pores small, closely 

 set in pairs, placed near each other above, and wider apart as they descend the sides. 

 At the under surface the ambulacral areas are large and smooth, and quite superficial. 

 The pores in the single ambulacrum are extremely small, and seen only in very good 

 specimens. 



The apical disc is narrow and elongated (fig. 1 b). The genital plates, four in 

 number ; are arranged in pairs, and between the anterior and posterior pairs one pair of 

 the five ocular plates is interposed by aposition of the discal elements. The apices of the 

 ambulacral areas are disjoined, as seen in fig. 1 b. 



The base is very flat (fig. 1 «), and the ambital border forms an acute angle. The 

 mouth-opening is lodged near the anterior border, in a depression of the test formed by 

 the arching round the angle of the anteal sulcus. The peristome has an oval figure, 

 slightly elongated transversely. 



The vent opens at the posterior truncated border, always low down, near the base 

 (fig. 1 d). The anal area is a very small, slightly hollowed-out space, the periprocte is 

 oval in a vertical direction, and has a well-defined border. 



The surface of all the plates is covered with an extremely fine abundant micro- 

 scopic granulation, which is preserved in patches on different parts of good specimens ; 



