﻿FROM THE CORALLINE OOLITE. 



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The inter-ambulacral areas are of unequal width, the anterior pair are the narrowest, 

 and the posterior pair and single area are the widest ; the former in b measures two 

 inches and four tenths, and the latter, which are about the same width, measure two inches 

 and nine tenths across. The long plates forming these areas are bent in the middle 

 (PI. XL), and their surface is covered with four rows of small, regularly arranged, 

 crenulated and perforated tubercles, raised on bosses, and surrounded by sunken areolas ; 

 the inter-tubercular portion of the plates is covered with close-set miliary granules. 

 (PL XXXIX, fig. 2.) 



The small apical disc is situated at the vertex, rather nearer the anterior than the 

 posterior border ; the discal elements are soldered together, and nothing but the four 

 ovarial holes, and small central madreporiform body are visible in the specimens I have 

 hitherto seen. 



In all the specimens of this urchin I have examined in different collections, the 

 under surface is covered with the Oolitic matrix, which adheres so firmly that it is 

 impossible to remove it ; the structure of the base is therefore unknown to me. M. Cotteau, 

 however, states, " That at the inferior surface, in his specimen, the ambulacra converge 

 in a straight line to the mouth ; they are narrow, bordered with pores, set wide apart, 

 and enclosed in very apparent depressions, which alternate with the elevations of the 

 inter-ambulacral areas." 



The anal opening is situated just below the posterior border, it has an oval form, its 

 long diameter corresponding to the antero-posterior diameter of the test. 



The mouth-opening, according to M. Cotteau, is excentral, nearer the anterior border, 

 the peristome is pentagonal, and surrounded with five prominent oral lobes. 



The test of this species is very thin and delicate, a circumstance which may account 

 for the fractured condition in which it is so often found ; in general it is met with in 

 masses of Oolitic limestone, from which it has to be cut out with great care. 



Affinities and differences. — This gigantic urchin so much resembles Pygurus PJiillipsii, 

 Wr., in all the leading points of its structure, that it is possible the latter may be only 

 a young form of Pi/gurus Hausmanni ; it requires, however, more specimens than I have 

 hitherto had at my disposal to state this as a fact. Pygurus Hausmanni in its magnitude 

 resembles Pygurus Icaunensis, Cotteau, but it differs from the latter in having its upper 

 surface more depressed, its ambulacral areas narrower, and in preserving their petaloidal 

 figure near to the border, whilst in P. Icaunensis they are wider in the upper half, and 

 much narrower in the lower half ; the base of this urchin is likewise more concave and 

 the inter-ambulacra more prominent and cushioned. P. Hausmanni differs from 

 Pygurus pentagonalis in having the dorsal surface more depressed, the general outline 

 more sub-circular, the ambulacral areas narrower above and wider below, and the single 

 inter-ambulacrum less rostrated than in the latter species. The prominence and elevation 

 of the ambulacra in Pygurus costatus with its pentagonal form readily distinguish it from 



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