76 HEMICIDARIS. 



and disproportionately small tubercles on the upper parts of the area ; apical disc large, 

 and composed of nearly equal-sized plates. 



Dimensions. — Antero-posterior diameter, one inch and five tenths. Height unknown. 



Description. — This beautiful species has been hitherto overlooked by English palaeon- 

 tologists, notwithstanding the very excellent figure given of it in the Transactions of the 

 Geological Society, 2d series, by the late Mr. Charles Stokes. I have not been able to 

 discover the original specimen, but through the kindness of Professor John Phillips, of 

 Oxford, I am enabled to figure a much better specimen, recently discovered by him at 

 Stonesfield. Unfortunately the upper surface of this specimen, like that figured by 

 Mr. Stokes, is alone exposed, the under surface being irremoveably surrounded by the 

 rock. 



The ambulacral areas are straight, and rather wide ; they are provided with two 

 marginal rows of small tubercles, set on slightly prominent elevations (fig. 3 b) ; a few small 

 miliary granules form incomplete circlets around their base; the under surface and 

 semi-tubercles are concealed ; but the apex is not much narrowed, and forms a rather 

 obtuse arch (in the figure the ambulacra is drawn rather too lanceolate), over the summit 

 of which the heart-shaped prominent ocular plates are rather conspicuously placed. 



The inter-ambulacral areas are nearly three times the width of the ambulacra ; the 

 primary tubercles are large and prominent at the equator of the test, but they suddenly 

 diminish in size at the upper part of the areas, so that the two upper tubercles of each 

 row are disproportionately small when compared with those at the equator. The bosses 

 of the large tubercles rise prominently from a narrow areola, and are sculptured with about 

 twenty crenulations at their summits ; the areola is surrounded with a complete circle of 

 small round scrobicular granules (fig. 3 b) ; the pores in the zones are small, and placed 

 widely apart, the rounded surface of the thick septa forming a moniliform line down the 

 middle of the zone. 



The apical disc is large (fig. 3 c) ; the genital plates are of nearly the same size ; the 

 madreporiform body is spongy and prominent, and occupies almost the whole of the surface 

 of the right anterolateral plate, which is the largest ; the genital foramina open at the 

 centre of mammillated elevations, near the apices of the plates ; there are two small 

 tubercles near the base of the postero-lateral and single plates, and four on the surface of 

 the left antero-lateral, as if the pair belonging to the right antero-lateral plate had been 

 transposed to its fellow of the left side, in consequence of the madreporiform body occu- 

 pying nearly all the surface of the right plate ; the anal opening is central and circular ; 

 the ocular plates form heart-shaped elevations around the circumference of the disc ; each 

 of the three anterior ocular plates support two small tubercles, but on the posterior pair 

 there is only one on each plate. 



The under surface of the test is unfortunately so much embedded in the rock, that its 





