FROM THE LOWER GREENS AND. 93 



rows of large tubercles, from twenty to twenty-five in each, according to the size of the 

 Urchin, all deeply crenulated and perforated, and gradually diminishing from the equator 

 to the apertures ; a single sinuous line of granules separates the tubercles, which are 

 placed closely together in the area (fig. 1 d). 



The poriferous zones are narrow at the base and sides, where the pores are arranged in 

 a single file (fig. 1 e); at the upper part they are bigeminal (fig. 1 cf), the double rows 

 encroaching on the width of the anibulacral area and diminishing the size of the tuber- 



cles therein. 



The poriferous plates are prolonged to the base of the tubercles in more or less apparent 



irregular sutures (fig. 1 d)- 



The inter-ambulacral areas are widely developed, the large plates support tubercles 

 closely resembling those of the ambulacra (fig. 1 y). In the figured specimen there are 

 six rows at the equator, and in larger specimens there are eight distinct rows. The 

 two internal rows have eighteen tul)ercles a little larger than the others, extending 

 from the peristome to the disc ; the other rows have a more limited range, and disappear 

 on the upper surface. It is only in the largest specimens that eight rows are found at the 

 ambitus, the tubercles of the shorter rows being a little less than those of the two internal 

 series (fig. 1 y) ; besides the primary tubercles a number of small secondary tubercles 

 are crowded along each side of the median suture, between the peristome and the 

 ambitus, and others occupy spaces by the side of the poriferous zones. The miliary zone 

 is wide, smooth, and depressed at the upper surface ; the granules are irregularly scattered 

 on its lower half, and some of them are even developed into small mammillated tubercles 

 on the npper surface; they form hexagonal circlets around the areas of the primary 

 tubercles ; the median suture is very well defined, and lies in a smooth depression of the 



test (fig. 1 n, c). 



The mouth-opening (fig. 1 d) is large and pentagonal, and the peristome divided into 

 lobes of unequal sizes; the arches that span the ambulacra are longer than those of the 



inter-ambulacra. 



The apical disc was very large ; the opening is pentagonal and acutely angular, the 

 angles extending far into the median suture of the inter-ambulacra (fig. 1 a andy). 



°The spines" are slender, and circular; above the milled ring of the head, there 

 is a short portion of the stem ornamented with fine longitudinal lines (fig. /), 

 whilst the portion beyond is entirely smooth. I have represented this character in the 



fragment fig. 1 /• 



M. Cotteau has figured a large example of this species from the Upper Ncocomian ; 

 from this we learn that age produces important modifications in the structure of the test ; 

 the poriferous zones arc very wide, and bigeminal, not only on the upper surface, but as 

 far down as the ambitus ; besides the eight rows of primary tubercles there are some 

 rudiments of secondary tubercles ; the miliary zone is wide and depressed at the upper 

 surface; the discal opening becomes more angular, and the ovarial plates penetrate 



