﻿ON THE ECHINOIDEA. 



453 



section of Cidaris Fowleri or C.jlorigemma. On the spur of the moment and with insuf- 

 ficient tools I rashly attempted to detach my treasure ; and as it turned out that the 

 lower portion of the test was fast imbedded in the hard rock beneath, I had the sad 

 mortification to fracture and destroy it. As I soon afterwards handed over to you the 

 fragments that remained, I now leave the description of them in your hands. This urchin 

 was associated, in the same rock-surface, with Pygaster semisulcatus, Terebratula simplex, 

 and other equally characteristic fossils of the Cheltenham Pisolite." 



Hemicidaris Brillensis, Wright, nov. sp. Supplement, PI. XLIII, fig. 2 a, b, c, d. 



Test sub-globose, ambulacral areas wide, with two rows of primary tubercles, which 

 extend over three fourths of the area ; inter-ambulacral areas narrow, with two rows of 

 small, nearly equal-sized, tubercles, ten in each row ; apical disc large, plates very narrow, 

 in consequence of the wideness of the vent ; mouth-opening large, peristome divided into ten 

 nearly equal lobes j poriferous zones narrow and much undulated ; pores very much crowded 

 at the base. 



Dimensions. — Height, one inch j transverse diameter, one inch and a half. 



Description. — This remarkable urchin, at first glance, resembles an Acrocidaris, from 

 the size, number, and development of the tubercles on the ambulacral areas. In most of 

 the other forms of Hemicidaris the semi-tubercles are limited to the basal region of the 

 ambulacra, but in this species they extend through nearly three fourths of the area ; this 

 region of the test is likewise much wider than in other congeneric forms, in which the 

 semi-tubercles are limited to the base of the area, and the margins thereof are occupied 

 by rows of small granules. There are about ten tubercles, very regularly arranged, in each 

 row j those at the base are very small, whilst the upper six pair are nearly of a uniform 

 size, although rather smaller than the primary tubercles of the inter-ambulacra ; the 

 upper part of the area has only a few, small granules on its margin. The poriferous zones 

 are very narrow, and extremely flexuous, winding round the border of the large semi- 

 tubercles, and only becoming straight at the upper fourth, where they cease (fig. 2 c). 

 The pores are separated by a thick septum, and there are six pairs of holes opposite each 

 inter-ambulacral plate ; they are much crowded together, in oblique rows, in the wide 

 spaces left by the small semi-tubercles at the base of the areas. 



The inter-ambulacral areas are narrow, scarcely twice the width of the ambulacra ; 

 they are occupied by two rows of primary tubercles, about ten in each row, of a 

 moderate size, and nearly uniform magnitude throughout (figs. 2 a, b) ; they are 

 raised on prominent bosses, with deeply crenulated summits (fig. 2 c) ; the areolas are 

 transversely oblong, and confluent above and below ; a double row of small granules 



