﻿FROM THE LOWER GREENS AND. 



253 



Trematopygus Faringdonensis, Wright, 1871. PI. LVII, fig. 1 a — h. 



Trematopygus Faringdonensis, Wright. In Phillips' Geology of Oxford, p. 434, 



1871. 



Diagnosis. — Test gibbous, oval, much inflated at the sides and base, narrow in the 

 anterior, and enlarged in the posterior third. Apical disc and vertex excentral and for- 

 wards ; ambulacra lanceolate, dorsal pores subpetaloid, and sulcus excavated out of the 

 posterior border ; vent pyriform, large; base concave, sides undulated by the inflation of 

 the inter-ambulacra ; mouth-opening large and oblique, and situated at the junction of 

 the anterior with the middle third. 



Dimensions. — a. Length one inch and three tenths ; height seven tenths of an inch ; 

 breadth one inch and two tenths, b. Length one inch and five tenths ; height seven 

 tenths ; breadth one inch and three tenths. 



Description. — The test of this rare Urchin has an oval outline, is a little narrower 

 before than behind, and is much inflated at the sides and base. The upper surface is 

 convex with the vertex excentral and forwards, fig. 1 a and b. 



The ambulacra are long, lanceolate, unequal, petaloid ; the posterior pair are much 

 longer than the others, and the single area is the shortest and narrowest ; at the under 

 surface the ambulacra form depressions, and the inter-ambulacra elevations, so that the 

 base is undulated at the sides and concave in the middle, fig. 1 c. 



The poriferous zones are well developed at the upper surface and the external rows 

 are slightly elongated, fig. lb; at the ambitus and base the pores are small and closely 

 biserial, and become larger and more conspicuous around the peristome, fig. 1 c. 



The apical disc is small and quadrate, fig. 1 a, b ; the four genital plates are perfo- 

 rated, and the anterior pair set closer together than the posterior pair ; the madreporiform 

 body occupies the middle of the disc and forms a prominent button there. The ocular 

 plates are very small and closely united to the other discal elements, see fig. 1 where the 

 disc is shown magnified six diameters. 



The periprocte is pyriform or oval, and acuminated at the upper extremity, fig. 1 

 a, b, and e ; it is quite supra-marginal ; the anal sulcus makes a deep indentation in the 

 posterior border, fig. 1 b, d, e, and from its sides two carinas proceed towards the base, 

 fig. 1 e. 



The tubercles are prominent and perforated, and raised on bosses surrounded by 

 depressed areas, the margins of which are encircled by granules, and all the inter- 

 tubercular surface is covered with a well-developed granulation, fig. 1 g ; at the base 

 the tubercles are larger and more spaced out, the mammelons are larger, and the granules 

 surrounding the areas more developed, fig. 1 h. In fig. 1 g an ambulacral area with the 



