BRITISH FOSSILS, 



3 



developed obtuse carination runs down the posterior ambulacral space, 

 with a curvature to a culminating point forming the superior margin of 

 the anus, and part of a prominent, and in some specimens apiculated, 

 arch over that aperture. The anus itself is vertically oval ; it varies in 

 size in different specimens, but is always small in proportion to the 

 body ; it occupies the upper portion of an obsolete and obscure groove, 

 which vanishes as it approaches the posterior margin. The anus is 

 always nearer the posterior margin than the apex. The ambulacra are 

 narrowly lanceolate and faintly petaloid ; they preserve the same pro- 

 portions in all the varieties. They are nearly equidistant from each 

 other at their summits. The number of pairs of pores in each row is 

 nearly equal in all, and varies from 28 to 30 in well-grown specimens. 

 The outer pores in each pair are elongated and oblique, and the two 

 pores are connected by a groove more or less distinct in different 

 examples. The ridges separating the grooves are rounded and smooth. 

 The ambulacral plates between the rows of pores are narrow, small, and 

 very numerous, but become much larger and broader suddenly, as the 

 ambulacra vanish. Each of these broader ambulacral plates has a pair 

 of minute pores, seeming to the eye as single, at its inferior and exterior 

 angle. The interambulacral plates are ample and oblong. Both sets 

 of plates are covered by minute moniliform tubercles, interspersed with 

 still smaller granules, and only to be distinctly seen in exceedingly well- 

 preserved specimens. There are four perforated ovarian plates, and 

 five oculars, with very distinct eye-pores. The two anterior pairs of 

 ovarian pores are placed nearer together than the two posterior ones, in 

 consequence of the anterior ambulacrum, which is narrow^er than the 

 others, only separating the former, whereas the latter are separated by 

 the summits of the two postero-lateral ambulacra. In consequence, 

 also, of the two antero-lateral ambulacra being rather broader than any 

 of the others, the lateral distance between each pair of ovarian pores is 

 greater than the distance between the frontal pair. The centre of the 

 ovarian disk is occupied by a distinctly marked punctated madreporiform 

 tubercle. The ventral surface is nearly plain, or very slightly convex. 

 The tubercles on it are larger than those of the dorsal. The mouth is 

 eccentric in the direction of the anterior extremity. It is pentagonal, 

 and surrounded by five prominent tubercles, which terminate the inferior 

 interambulacral spaces. These tubercles are curiously granulated in a 

 reticulate fashion. From the mouth radiate ten short petaloid ambu- 

 lacra, constructed like those of the dorsal surface. 



The following table exhibits the comparative dimensions of six speci- 

 mens, five of which come from the same locality (green sand of War- 

 minster), and one (No. 4) from the junction beds of the green sand and 

 chalk marl at Maiden Bradley, Wiltshire, in which the majority of 



