FROM THE INFERIOR OOLITE. 367 



portions of tlie inter-ambulacra ; apical disc central ; poriferous zones petaloid on the 

 upper two thirds of the dorsal surface ; lower third, and basal portion, with parallel rows 

 of unconnected pores ; anal valley long, narrow, deep, extending from the disc to the 

 border, fissure-like above, and slightly expanded near the margin ; single inter-ambulacrum 

 much deflected and beak-like, truncated at its extremity ; mouth-opening small, sub- 

 central, lodged in a deep depression, peristome surrounded by five oral lobes. Greatest 

 diameter of the shell across the middle of the postero-lateral inter-ambulacra. 



Dimensions. — Height, one inch and one fifth ; antero-posterior diameter, two inches 

 and six tenths ; transverse diameter, two inches and nine tenths. 



Description. — This beautiful Clypeus attains considerable dimensions, and preserves 

 through all the phases of its development the specific characters enumerated in our 

 diagnosis. It is remarkable for having the upper surface evenly convex, much elevated, 

 or even sub-conical ; it is likewise broader across the postero-lateral ambulacra than it is 

 long, its base is more concave, the inter-ambulacral segments of this region are more 

 tumid, and the outline of the basal margin more undulated (fig. 1 c, d), than in any other 

 species. 



The ambulacral areas are narrow at the margin, slightly enlarged at the upper third, 

 and lanceolate at the apex. The poriferous zones in the upper two thirds of the dorsal 

 surface are slightly petaloid; in the lower third the pores are small, parallel, equal, and 

 unconnected (fig. \ a, c); in the petaloid portion, the pores of the inner row are round, 

 those of the outer row are slit-like and oblique (fig. 1 e) ; the ambulacral plates are very 

 narrow, there being seven, with a corresponding number of pairs of holes, opposite each 

 large inter-ambulacral plate (fig. 1 e). 



The inter-ambulacral areas are of unequal width, the anterior pair are the narrowest, 

 the posterior pair the widest, and the single area of intermediate dimensions ; they 

 are almost uniformly convex on the upper surface, and present very great inequalities at 

 the base, more so, in fact, than in any other species of the EcHiNOBRissiDiE. The single 

 inter-ambulacrum is short, curved, much deflected, and truncated ; it is deeply cleft by the 

 anal valley, which extends from the apical disc to the margin ; this sulcus is in the form of 

 a deep fissure, with vertical parallel walls in the upper half of its length, but moderately 

 expanded in the lower half (fig. 1 a and d). 



The small, narrow, and elongated apical disc is well preserved in most of my 

 specimens, and is situated near the centre of the test, immediately behind the vertex; it 

 is composed of two small, anterior, perforated genitals (fig. 1/), and two larger posterior 

 perforated genitals, with a single, long, imperforate genital plate, which descends into the 

 narrow anal valley ; the spongy madreporiform body rises from the surface of the right 

 anterior genital, and extends into the centre of the disc ; the five small ocular plates 



48 



