FROM THE STONESFIELD SLATE. 77 



removal therefrom is impossible ; we are, therefore, in ignorance about many important 

 points relating to the anatomy of this species. 



Affinities and differences. — Ilemicidaris Sfokesii resembles Hemicidaris pushdosa and 

 Hemicidaris diademata, in having straight ambulacral areas and disproportionately small 

 tubercles on the upper parts of the inter- ambulacra. From Hemicidaris pustulosa it is 

 readily distinguished by its regular rows of small marginal tubercles in the ambulacral 

 areas, with miliary granules between them, and in having small, single, primary tubercles 

 only on the upper parts of the inter-ambulacral areas, but no clusters of granules thereon, 

 as in Hemicidaris pustulosa. The test likewise is much more depressed, and the apical 

 disc is not so prominent. Professor Agassiz observes* of Hemicidaris diademata, that 

 the essential character consists in the rapid diminution of the large tubercles on the 

 upper part of the inter-ambulacral areas ; a unique peculiarity in this genus, forming a 

 remarkable contrast to the exuberance of these same tubercles in other species of 

 Hemicidaris. The discovery, however, of Hemicidaris pustulosa and Hemicidaris Stolcesii, 

 show that this character is shared in common with other congeneric forms. From 

 Hemicidaris diademata this species is distinguished by having the ambulacral areas 

 straighter, the tubercles on the upper parts of the inter-ambulacral areas larger ; and the 

 miliary granules are likewise larger and less numerous than those which cover the plates 

 in Hemicidaris diademata ; but in other respects, as far as I can make a comparison, 

 there is a very close affinity between these species. 



Locality/ and Statigraphical range. — This species was first collected by the late 

 Mr. Charles Stokes, from the Stonesfield Slate at Stonesfield; and the urchin I figure 

 was collected from the same rock and locality by Professor John PhiUips, who has kindly 

 communicated the specimen for my monograph. In the Stonesfield Slate at Eyeford, 

 Gloucestershire, a portion of a Hemicidaris has been occasionally found, which belongs 

 likewise to this species. When Ave consider the enormous surface of this rock which is 

 annually exposed by the splitting of the same into slates, and the very few specimens 

 that have been found during the many years the quarries have been worked, we must 

 consider Hemicidaris Stokesii as one of the rarest species of our Oolitic Urchins. 



History. — First figured by Mr. Stokes, in the Transactions of the Geological Society ; 

 as the type specimen appears to have been unknown, and as it was not named by its dis- 

 coverer, it was not entered in our lists of species. It is now described for the first time. 

 The specimen figured belongs to the Oxford Museum ; I have prepared plaster moulds 

 from the same for the British Museum, Geological Museum, Jermyn Street, and the 

 Bristol Institution. 



* • Ecliinodermes Fossiles de la Suisse,' part ii, p. 49. 



