FROM THE LOWER GREENSAND. 353 



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



Trematopygus Faeixgdonensis, Wright. In Phillips' Geology of Oxford, p. -134, 



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 snbpetaloid, 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 ; 

 lireadth one inch and two tenths, h. 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. \ h ; 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, h ; 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. \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. Iff; at the base 

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

 surrounding the areas more developed, fig. 1 h. In fig. 1 y an arabulacral area with the 



