FROM THE GREY CHALK. 123 



at the base and upper surface ; they are feel)ly developed throughout, and finely perforated 

 at the summit ; the areote are regularly spaced out and placed on the border of the poriferous 

 zones, which are straight, slightly depressed, and formed of small round pores, disposed 

 in single oblique pairs throughout, from the peristome to the disc ; the pairs of pores 

 are separated from each other by a horizontal granular ridge of the test, which is more or 

 less apparent in diff'erent s])ecimens ; in some examples one of the rows of tubercles 

 becomes abortive ; the surface of the area is filled in with numerous small granules, and the 

 ambulacral plates are shghtly marked with impressions on the line of the median suture. 

 The inter-ambulacral areas, fig. 2 c/, are double the width of the ambulacral, and provided with 

 two rows of tubercles, similar in structure to, but larger in size than, those of the ambulacra; 

 the areolae occupy the centre of the plates, and are surrounded with a close-set granulation ; 

 two of the granules, elongated in a vertical direction, unite the contiguous areolae, which 

 imparts a moniliform character to well-preserved tests of this pretty little Urchin ; the plates 

 are marked with impressions more or less deep at the inner and outer angles, and on each 

 side of the vertical filament at the lower part of each plate (fig. 1 c, fig. 2 c). 



The mouth-opening is small, situated in a slight depression, and the peristome is 

 delicately incised into ten unequal lobes (fig. 2 a). 



The vent is large, sub-elliptical in shape, and contracted behind ; the apical disc 

 forms a narrow, elongated, subpentagonal ring, somewhat peculiar in its structure, for 

 instead of the small ocular plates being wedged in between the larger genitals, they 

 are arranged alternately with them on the same fine, and form a strong ring around the 

 elliptical vent ; the genital plates are a little larger than the oculars, and have two small 

 tubercles on their surface ; the oculars are covered with a fine granulation, and the 

 madreporiform tubei'cle is conspicuous by its spongy surface (fig. 2 h). 



This Urchin varies much in size ; the specimens figured by Goldfuss, Desor, and 

 Dixon, with those I possess, are all small. M. Cotteau has given elaborate details of a 

 larger specimen found in the Department of the Sarthe, in which the characters of the test 

 are admirably exhibited ; they are beautifully figured, both in his fine plates on the Echinides 

 of the Sarthe, and likewise in those in the ' Paleontohigie Eran9aise,' some of which I have 

 copied in PI. XXIX b, as none of my specimens have the characters so well preserved as 

 in the ])erfect fossil test figured by my friend. 



Jffiiiities and Differences. — Gbjpliocyphas radiatus is so rare an Urchin in the English 

 Chalk that it is not likely to be mistaken for any other. It is smaller and more globular 

 than Ecliinocyphus difficilis, which it most resembles, and has the tubercles perforated, the 

 miliary zone wider and more granular, the disc smaller and more solidly united to the 

 coronal places than in any Ecliinocyphi. 



From a careful examination of the ample materials at his disposition, M. Cotteau con- 

 cludes that the large examples forming the type of the species represent EcJiinopsis 

 contexta, Ag., and exhibit natural impressions either deep and angular or linear and 

 attenuated. Some specimens, less inflated, and possessing a stronger development of the 



