100 HEMICIDARIS. 



right plate; the postero-lateral and single plates are nearly alike in size, but the single 

 one is the smallest ; all the genital holes are marginal ,■ as the anal opening is transversely 

 oval, and enlarged at their expense, that aperture is excentral. The ocular plates are 

 small, heart-shaped bodies, wedged in between the truncated apices of the ambulacra and 

 the genital plates ; the eyeholes are quite marginal. 



The spines (fig. 4 c, d) are sub-cylindrical, and slightly compressed on the sides ; the 

 diameter of the largest spine is about one tenth of an inch ; they are all so much weathered, 

 that the longitudinal lines on their surface cannot be seen ; above the prominent milled 

 ring (fig. 4 c) there is a broad, well-defined space, marked with longitudinal lines, 

 which forms an important diagnostic mark between this species and its congeners. The 

 articulating head is small, and the acetabulum diminutive. 



In none of the specimens is the mouth opening exposed. 



Affinities and differences. — Hemicidaris Purbeckensis differs from Hemicidaris inter- 

 media in having smaller tubercles and less prominent bosses. In this respect it is allied to 

 the Hemicidaris Bravenderi (PI. V, fig. 1 ; PI. XI, fig. 3) ; but the character of the spines, 

 with the striated space above the milled ring, distinguishes it from both. Its closest affinity 

 is with Hemicidaris Davidsoni ; but from that Portland species it is distinguished by 

 having narrower inter-ambulacral areas, smaller bosses, more confluent areolas, and above 

 all by the comparative regularity of the semi-tubercles, which, on the contrary, extend 

 singly up nearly half the ambulacral areas in Hemicidaris Davidsoni (PI. IV, fig. 2 b, c). 

 The apical disc is likewise much smaller, and the vent not so wide. 



Hemicidaris stramonium, Agassiz. 



Hemicidaris stramonium. M'Coy, New Mesozoic Radiata, Annals and Magazine of Natural 



History, vol. ii, new series, p. 420. 



— — Forbes, Memoirs of the Geological Survey, Decade III, notes 



on Hemicidaris. 



— — Morris, British Fossils, 1854, 2 ed., p. 82. 



From the doubtful manner in which this species was quoted by Professor Forbes, in 

 his 3d Decade of the Memoirs of the Geological Survey, I requested Professor Sedgwick 

 to permit me to examine the type specimens belonging to the Geological Collection 

 of the University of Cambridge, a favour which the learned professor most kindly and 

 readily granted. These urchins had been labelled by Professor M'Coy Hemicidaris 

 stramonium, Agassiz, and were catalogued as such in the Addenda to his Paper on some 

 new Mesozoic Radiata, published in the ' Annals of Natural History.' On examination, 

 these urchins proved to be two small imperfect specimens of Hemicidaris intermedia, 

 from the Coral Rag, Calne. On placing these specimens side by side with a true 

 Hemicidaris stramonium, Agass., kindly sent me by M. Michelin, the difference between 



