2 



BRITISH FOSSILS. 



their length, small secondary perforated tuhercles on minute hosses, 

 one to each plate, with scattered granules between them. On the 

 lower third of the ambulacral areas these suddenly increase in size 

 to twice the dimensions of those above. The large ambulacral tubercles 

 are diffusely arranged, so as to alternate with considerable interspaces. 

 The avenues of pores are slightly undulating. About nine pores 

 correspond to an interambulacral central plate. The mouth in our 

 only specimen is concealed, and the apical disk not preserved. 



The spines are quite smooth, slender, and subcylindrical, slightly 

 compressed at the sides. There is a short longitudinally striated well 

 defined space above the swollen crenated neck. The articulating base 

 is small. 



The diameter of the specimen is one inch and two-twelfths. The 

 diameter of the largest spine found is one- tenth of an inch. 



Affinities and Differences, — Hemicidaris Purheckensis differs from 

 H. intermedia and crenularis in having smaller and more numerous 

 tubercles, in proportion to the size of the body, on the interambulacral 

 areas, and a larger granulated intermediate space in their centres ; 

 the large tubercles on the ambulacral areas, instead of being closely 

 set as in both the species named, are scattered and alternating; 

 the number of pairs of pores opposite each large interambulacral plate 

 are fewer. The ambulacra are nearly straight, instead of being undu- 

 lated. Above all, the spines are quite distinct in their characters. 



It differs from H. stramonium^ a Portland oolite species, said by 

 Mr. M'Coy to occur in England, in having a greater number of large 

 ambulacral tubercles, wider ambulacral areas, and much smaller inter- 

 ambulacral tubercles. 



From H. mitra, also a foreign Portland oolite species, to which, like 

 the last, it approaches in the straightness of the ambulacra, it differs 

 in the arrangement of the interambulacral tubercles. 



H. diademata and Thurmanni, foreign Kimmeridge species, have 

 fewer interambulacral tubercles. The peculiar structure of the ambu- 

 lacra of H. alpina sufficiently distinguish that species. 



Since, except in the cases of intermedia and crenularis^ the spines of 

 the species I have mentioned are unknown, it is probable that when they 

 shall have been found, the differences indicated will be accompanied by 

 still more important distinctions. 



Locality and Geological Position. — This Hemicidaris is peculiarly in- 

 teresting as being the first Echinoderm ever found in the Purbecks. 

 It was discovered by myself at Swanage in Dorsetshire, in December, 

 1849. For several days I had found the 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 



