﻿FROM THE GREAT OOLITE. 



393 



the length of which gradually diminish from the middle of the petal upwards to the 

 apex, and downwards to about the lower fifth of the area, where the oblong pores are 

 reduced to simple pores, like those of the inner row (fig. 2d); at the border the pairs of 

 pores lie close together, at the base they are much wider apart (fig. 2 b), and about half 

 an inch from the mouth they again greatly increase in number, and in the depressed 

 phylloidal floscule form a regular series of triple oblique pairs (fig. 2 b), with twelve rows 

 in each zone. 



The inter-ambulacral areas are uniformly convex above, the postero-lateral and single 

 inter-ambulacrum are of the same width, and the anterior pair are narrower at the under 

 surface ; these segments form prominent cushions, which are very tumid at the border, but 

 less so near the mouth ; the most prominent part of each cushion is flattened at the 

 interspace between the ridges which radiate from the disc to the mouth. 



The single inter-ambulacrum is produced, rostrated, and deflected ; at its infra-marginal 

 border is a deep anal depression, with prominent and inclined sides, at the bottom of 

 which the elliptical anus opens (fig. 2 b) ; the anal valley indents the border, and forms a 

 conspicuous notch in the margin when the test is viewed from behind forwards, as is well 

 shown in fig. 2 c, which likewise exhibits the undulations of the base. 



The apical disc is remarkable for the size of the madreporiform body and the 

 smallness of the genital plates (fig. 2 /) ; the anterior pair are less than the posterior pair, 

 and the four are perforated ; the single plate is posterior, and imperforate (fig. 3) ; the 

 small rhomboidal ocular plates alternate with the genitals, the very minute eyeholes are 

 perforated in the centre of the plates ; the madreporiform body rises from the surface of 

 the right antero-lateral genital, extends over the surface of all the others, and occupies the 

 centre of the disc (fig. 3). 



The large mouth-opening is sub-central and forward ; the peristome is pentagonal, 

 and surrounded by five prominent oral lobes (fig. 2 b) ; alternating with them are the five 

 depressed phylloidal terminations of the ambulacra, in which numerous pores are arranged 

 in triple oblique rows ; this crowding together of the pores in regular order, imparts 

 an ornamental character to the five oral ambulacra. 



The tubercles on the upper surface are very small, on each plate they are arranged in 

 four or five tolerably regular horizontal rows, and surrounded by sunken areolas ; 

 the inter-tubercular surface is covered with microscopic granules, placed so close together 

 that all the intermediate portion of the plates, when examined with a low magnifying 

 power, is seen to have a finely sculptured appearance. At the base the tubercles are 

 larger, and disposed with less regularity ; they increase in size, and are set closer together 

 at the border and on the convex surface of the five basal cushions, whilst they are still 

 larger and placed wider apart near the mouth-opening and on the sides of the ambulacra. 



Affinities and differences. — This species has been frequently confused with Pygurus 

 depressus, Ag., which it very much resembles ; according to M. Cotteau, it is distinguished 



