FROM THE UPPER GREENSAND. 313 



Holaster obliquus, nov. sp., Wright. PI. LXXVIII, fig. \,a — e. 



Diagnosis. — Test oblong, slightly cordate, greatest width across the anterior third, 

 gradually diminishing in size to the posterior border, which is narrow and obliquely 

 truncated ; ambulacral summit excentral near the junction of the anterior with the 

 middle third, upper surface convex and sloping downwards to the posterior margin ; 

 anal opening in the upper part ; periprocte elliptical ; area narrow and inclined downwards 

 and inwards. Base rounded ; mouth-opening near the anterior border ; anteal sulcus 

 shallow and inclined downwards and inwards ; peristome small, transversely oval, 

 unilabiate, and situated in a depression. 



Dimensions. — Antero-posterior diameter two inches ; transverse diameter one inch 

 and three tenths ; vent above the base seven tenths of an inch. 



Description. — This rare Urchin has an oval outline ; its greatest width is posterior to 

 the antero-lateral ambulacral areas. From this part of the ambitus it tapers obliquely 

 inwards towards the narrow posterior border. The upper surface is convex and the 

 ventral summit, near the apical disc, is situated about the junction of the anterior 

 with the middle third, and to this point all the ambulacral areas converge ; from 

 the disc to the posterior border the upper surface inclines obliquely downwards and 

 forms a long slope towards the anal area. The antero-lateral ambulacra are short, 

 straight, apetaloid, and lanceolate, and pass straight upwards from the ambitus to the 

 disc. The postero-lateral ambulacra are straight, lanceolate, and apetaloid ; they are 

 much larger than the anterior pair from the excentral position of the summit. The 

 poriferous zones are narrow and superficial ; the pores small, and placed in pairs ; forty 

 pairs exist between the disc and the ambitus, and below that line they become invisible. 



The apical disc is a small, narrow, oblong body, of which the elements are so 

 closely soldered together that the sutures are obliterated. 



The mouth-opening is small, and situated at a short distance from the anterior 

 border in a depression formed by the incurving of the anteal sulcus. The peristome 

 is small and transversely oval, and is placed near the junction of the first with the second 

 fourth part of the total basal length of the shell. 



The vent opens in the upper part of the posterior border ; the anal area is very 

 narrow, and the oval periprocte is situated high up in this space, where the convex portion 

 of the long, obliquely-sloped back bends over the truncated portion of the posterior 

 border. 



The surface of the plates is covered with a close-set miliary granulation, which is 

 beautifully preserved in a specimen from the Upper Greensand of Wiltshire. On the 

 larger plates a few small tubercles are developed without much order in their arrange- 

 ment. Six to eight such tubercles are seen upon many of these plates. 



