FROM THE WHITE CHALK. 303 



metres ; transverse diameter 42 millimetres ; height 26 millimetres. Var. mvnor. — 

 Anteroposterior diameter 27 millimetres; transverse diameter 25 ; height 15. 



Description. — This elegant Urchin has a regularly cordate ambitus. It is deeply 

 channelled in front by the anteal sulcus, which has sloping walls, with angulatcd borders 

 above, and a deep granulated channel below. The greatest breadth of the test is 

 immediately behind the antero-lateral ambulacra ; from this point it slopes gently 

 towards the posterior border, which is very narrow, and slightly truncated downwards 

 and inwards. 



The upper surface is convex, very much rounded before, and declining gently behind. 

 The under surface is very much depressed near the mouth, and slightly convex in the 

 single inter-ambulacrum, which has a sinuous ridge along the middle line, with nodular 

 elevations at each alternate angle. The vertex is nearly central. The anteal sulcus is 

 deeply hollowed out from the summit to the mouth, especially at the ambitus, and is 

 circumscribed laterally and superiorly by angulated carina?. The single ambulacrum is 

 lodged in the sulcus, and composed of very small round pores, separated by a granuliform 

 elevation, and situated in a small, smooth, lanceolate fossa, very narrow above and 

 expanded below. The pores are close together above, and widen as the plates become 

 larger. There are about twenty-eight plates visible in a full-grown specimen. The 

 antero-lateral ambulacra are subpetaloid in their upper part, and unsymmetrical, in 

 consequence of the pores of each pair being wider apart in the outer row than in the 

 inner, and, moreover, being united by a deeper groove. There are about twenty-four 

 pairs of pores in the subpetaloidal portion. They are closely approximated, and those 

 that are below widen out ; the outer series is likewise slightly arcuated. In the postero- 

 lateral ambulacra the two series are nearly similar, and there are about eighteen pairs of 

 pores in the subpetaloid portion. There are about nine plates in each series in the 

 dorsal portion of the lateral inter-ambulacral segments, and ten above the vent in the 

 posterior segment. The infra-anal portion of the latter is composed of two series of five 

 irregularly polygonal plates each. They are remarkable for presenting gibbosities at 

 regular intervals. In the inferior portions of each postero-lateral ambulacrum there are 

 seven plates in each series. These are polygonal, large, and elongated, and bear the 

 pairs of pores, which are minute and oblique, close to their inner angles. The pores of 

 the ambulacra around the mouth form an obscure star. 



The surface of the plates is profusely covered with very fine granules, which observe 

 a very regular arrangement among them. Two sets of tubercles of unequal size cover 

 the plates at regular distances apart ; the small tubercles, which are seated among the 

 granules of the upper surface (fig. 2/), and the large tubercles, which are perforated and 

 crenulated, are seen on the upper surface around the apical disc, upon the middle of the 

 single inter-ambulacrum, and upon the borders of the anteal sulcus (fig. 2 e). In the in- 

 fra-marginal region and along the central space on each side of the sinuous ridge (fig. 2 b), 

 they are large and conspicuous. Each of the large tubercles is elevated upon a boss 



