FROM THE LOWER CHALK. 323 



Holaster Trecensis, Leynterie. PI. LXXIV, fig. 2, a — e. 



Holaster trecensis, Leymerie. Mem. de la Soc. geol., p. 2, pi. ii, fig. 1, 1842. 



— — Agassiz. Catal. raisonne, p. 134, 1847. 



— piLLULAvar. maxima, Agassiz and Desor. Ibid., p. 135, 18 47. 



— trecensis, (V Orhigny . Prodrome 2, p. 269, No. 1161, 1847. 



— — Pal. Franc., Ter. Cretaces, vol. vi, p. 101, 



pi. 847, 1853. 



Diagnosis. — Test oblong, cordiform ; upper surface very much elevated, rounded, and 

 grooved before, tapering gently to a narrow truncation behind. Ambulacral summit 

 nearly central ; under surface very flat ; mouth near the border. Vent in a small 

 marginal anal area near the angle ; upper surface covered with large tubercles, arranged 

 without much regularity upon both areas. 



Dimensions. — Antero-posterior diameter two inches and four tenths ; transverse 

 diameter two inches and four tenths ; height one inch and five tenths. 



Description. — The test is oval and cordiform, much inflated, and nearly as broad as 

 long ; enlarged and sinuous before, accuminated behind. The upper surface is much 

 elevated and very convex, the outlines antero-posteriorly, as well as laterally, describing 

 regular curves (fig. 2 c, d). The ambulacral summit is nearly central (fig. 2 a), where 

 we find the apical disc, which is small, and shows four perforated genital plates. The 

 central portions of the inter-ambulacra are rather prominent, which imparts an angular 

 appearance to the ambitus (fig. 2 d). The base is very flat, with the exception of a 

 slight prominence in the middle of the plastron, where we observe seven alternate 

 elevations, with a connective zigzag ridge between (fig. 2 b). The anterior portion near 

 the anteal sulcus is slightly depressed (fig. 2 b), where we find the mouth-opening, which 

 is large, and transversely oval. The peristome is bilabiate; the anterior lip is depressed 

 and convex, and the posterior elevated and prominent. The vent occupies the posterior 

 border, and opens near the base ; it is situated in a small triangular anal area, slightly 

 excavated out of the border (fig. 2 e). The periprocte is longitudinally oval, placed under 

 an angular projection in the median line, with two lateral ridges to divide the area from 

 the ambital circumference (fig. 2 e). 



The ambulacral areas are very feebly defined in this Urchin ; the anterior single area 

 is lodged in the sulcus ; its poriferous zones are only slightly visible near the summit. 

 The antero-lateral and postero-lateral pairs are narrowly lanceolate. The anterior pair 

 curve slightly forwards, and the posterior pair are straight (fig. 2 a). The poriferous zones 

 are formed of transverse oblong pores arranged in pairs, and the anterior is smaller than 

 the posterior row ; they extend half-way over the upper surface and sides, and disappear 

 above the ambitus (fig. 2 c, d). 



The tubercles are of two kinds ; small tubercles, which are seen in the ambulacral 



