FROM THE MARINE PURBECK BEDS. 99 



Dimensions. — Specimens all more or less distorted. Height, about three quarters of 

 an inch ; transverse diameter, about one inch and three tenths. 



Description. — The discovery of Echinoderms in the Cinder Bed of the Purbecks was 

 one of the rewards of the careful examination of these strata made by the officers of the 

 Geological Survey of Great Britain. "For several days," observed Professor Forbes,* who 

 found the first specimen at Swanage, in Dorsetshire, " I had found spines of an urchin 

 with which I was unacquainted among the marine fossils which occur in a zone on the 

 summit of the well-known ' Cinder Bed,' composed chiefly of Ostrea distorta, Sow., and 

 constituting a conspicuous stratum in the middle division of the Purbecks. A careful 

 search, during which I was rewarded by the discovery of several new forms of marine 

 Purbeck Mollusca, resulted in the finding of a very perfect specimen of the body of the 

 Hemicidaris, now first described, accompanied by its spines, identical in structure with 

 those previously observed." It is this same urchin, with others since found, that my 

 figures represent. 



The body is sub-globose, but was apparently rather depressed above ; the specimens 

 being all more or less distorted, it is impossible to describe its form with accuracy. 



The ambulacral areas are narrow, and only slightly undulated ; there are two rows of 

 minute perforated tubercles, on miniature bosses, on the margins, about sixteen in each 

 row ; a zigzag line of very small granules runs down the middle of the area, and sends 

 small branches of granules to encircle each minute tubercle (fig. 4 b) ; at the base of the 

 area there are five pairs of small semi-tubercles, which, in a small specimen before me, have 

 a very regular arrangement but in the larger specimens are more diffusely disposed, so as 

 to alternate with considerable interspaces. The poriferous zones are slightly undulated, 

 the pores are small, the pairs are a little oblique, and there is a slight elevation of the 

 test corresponding to the septa ; there are about nine or ten pairs of pores opposite 

 one of the large inter-ambulacral plates. 



The inter-ambulacral areas at the equator are hardly three times the width of the am- 

 bulacral ; the two rows of primary tubercles occupy the centre of the plates (fig. la); 

 the tubercles are small, set upon a smooth, slightly elevated boss, with a deeply crenulated 

 summit; around the base is a smooth, well-defined, and grooved areola; the ambulacral 

 and centro-sutural sides of the large plates are bordered by small, rounded granules 

 (fig. 4 b), some of which extend between adjacent areolas in the upper part of the areas, 

 but they are absent from the plates below when the areolas are confluent. The areola is 

 wide in comparison with the size of the boss (fig. 4 b ). There are about eight primary 

 tubercles in each row, gradually increasing in size as they approach the equator. 



The apical disc is moderately large (fig. 4 a) ; the antero-lateral genital plates are 

 much the largest ; the madreporiform body occupies nearly all the surface of the 



* 'Memoirs of the Geological Survey,' Decade HI, description of pi. v, p. 3. 



