FROM THE GREAT OOLITE. 239 



vent large, excentral ; mouth large 5 peristome decagonal, lobes nearly equal ; notches 

 slightly incised. 



Dimensions. — Height, nearly four tenths of an inch ; transverse diameter, seven tenths 

 of an inch. 



Description. — This beautiful little urchin is so admirably preserved in the Corn- 

 brash of Wiltshire, that it forms one of the most characteristic fossils of that formation ; 

 the test is small, sometimes circular, oftener sub-pentagonal, and always much depressed 

 (fig. 3 c). 



The ambulacral areas are narrow and prominent (tig. 3 a, b) ; as the poriferous zones 

 are wide, for so small an urchin, the ambulacra appear isolated, and the upper surface of 

 the test has a radiated appearance, which is increased by the size and smoothness of the 

 apical disc (fig. 3 a) ; the ambulacra converge in straight lines from the base to the disc ; 

 they are furnished with two rows of small tubercles, which, although microscopic in size, 

 have crenulated bosses and perforated summits ; the tubercles are disposed with great 

 regularity on the borders of the areas, at a moderate distance apart (fig. 3 c, d), so that 

 there are only from ten to twelve in each row ; in the middle of the area a double row of 

 granules separates the tubercles, and lateral rows encircle the areolas, which are only open 

 on the sides (fig. 3 d). The poriferous zones are wide and straight; the pores are large, 

 and disposed obliquely in single pairs (fig. 3 d), forming a rectilinear file on each side of 

 the ambulacral segments ; there are from five to six pairs of pores opposite each large 

 tubercular plate. 



The inter-ambulacral areas are three times the width of the ambulacral ; the two rows 

 of tubercles, about eight in each row, are nearly equidistant throughout (fig. 3 a) ; the 

 two basal pairs are small (fig. 3 b), the three equatorial pairs large (fig. 3 c), and the 

 three upper pairs suddenly diminish in size and become dwarfed as they approach the 

 disc (fig. 3 «, c) ; two or three pairs of tubercles at the equator have very large bosses (fig. 

 3 e), whilst all the others are of moderate size; they have narrow elliptical areolas, 

 of which the larger are confluent (fig. 3 d); the miliary zone consists of two rows of 

 granules, which form scrobicular circles around the areolas (fig. 3 d) ; as the rows diverge 

 above, a small triangular space on the upper part of the segment is left naked (fig. 3 a) ; 

 the areolas are likewise separated from the poriferous zones by a single row of granules 

 (fig. Sd). 



The apical disc is large and pentagonal, about two fifths the diameter of the test 

 (fig. 3 a) ; the anterior and posterior pair of genital plates are nearly alike in size and 

 form (fig. 3/); the oviductal holes are perforated near the external third of the plates; 

 the posterior single plate projects a little further into its corresponding segment than the 

 others, it is largely excavated for the vent, of which it forms the posterior wall (fig. 3/) ; 

 the sur-anal plate is central, single, and pentagonal, and forms the anterior wall of the 

 vent (tig. 3/), its sides being formed by the posterior pair of ocular plates, which are 



