FROM THE CORALLINE OOLITE. 399 



of the lobes are fewer in number, and larger in size (fig. 1 b), than those of the dorsum, and 

 their surface is perforated; the tubercles on the convex portions of the inter-ambulacra are 

 very small, and placed so close together that the borders of the areolas form hexagonal 

 figures ; the basal ambulacra are destitute of tubercles, and the plates are covered only 

 with miliary granules ; the nakedness of the surface clearly displays the size of the plates, 

 and their peculiar figure, with the distant pores in the narrow zones in this region of 

 these segments. 



The oblong vent is infra-marginal, and the anal valley is shallow, with sloping sides, 

 on which some larger tubercles are disposed. 



Affinities and differences. — This species resembles Pygurus orhiculatus, Leske, so 

 beautifully figured by M. Cotteau ; Pygurus costatus, however, has a much larger test, it 

 is more depressed at the upper surface, and has more prominent, costated, ambulacra ; the 

 pentagonal border, is more rostrated behind, and more emarginate before ; the larger mouth- 

 opening has more prominent lobes and larger phylloidal ambulacra, and the test presents 

 an ensemble of characters by which these two allied forms may readily be distinguished. 



Pygurus costatus resembles Pygurus Marmonti, Beaud., from the Kelloway ferrugineux 

 of the Sarthe, in the general disciform shape of the test and flatness of the base, but in 

 the specimen of P. Marmonti before me the poriferous zones are petaloid to the 

 border, the vent is removed inwards some distance from the margin, and the test has, 

 moreover, an orbicular circumference. 



The depression of the dorsum, thinness of the border, angularity of the sides, flatness 

 of the base, and prominence of the dorsal ambulacra, clearly distinguish Pygurus costatus 

 from its other Oolitic congeners, and I am unacquainted with any other foreign form 

 beside those enumerated with which to compare our urchin. 



Locality and StratigrapJiical position. — This species was collected from the Lower 

 Calcareous Grit of Oxfordshire and Wiltshire ; the specimen I have figured was found near 

 Oxford, and I have another from the same rock near Calne. It is not a common 

 species, as I have rarely seen it in collections of Calcareous Grit fossils. The specimen 

 I have figured was most kindly given to me for this work by my friend, the Rev. 

 P. B. Brodie. 



52 



