98 HEMICIDARIS. 



plates are heart-shaped ; the eyeholes are small and marginal ; the posterior pair extend 

 into the anal circle in consequence of the diminution in size of the postero-lateral and 

 single genital plates (fig. 2/). 



The base is fiat, the mouth opening is large, and occupies three fifths of this region ; 

 the peristome is deeply notched, the lobes are nearly of equal size, and those of the ambu- 

 lacral areas are the largest (fig. 2 e). 



The primary spines are not long, rarely exceeding in length one and a half times the 

 diameter of the test ; they do not taper suddenly ; the head is short ; the acetabulum is 

 small, crenulated on the rim, and surmounted by a prominent milled ring, beyond 

 which is a second ring-like elevation ; the surface of the stem is covered with very fine 

 longitudinal lines, which are almost always effaced ; the secondary spines, articulated with 

 the minute tubercles of the ambulacra, and those surrounding the margins of the inter- 

 ambulacral plates, are short, thorn-like processes. This description of the spines is made 

 from French specimens. 



Affinities and differences. — This urchin resembles very much Hemicidaris Purheckensis, 

 both in its general configuration and in many points of its organization ; but differs 

 from it in the manner the semi-tubercles are arranged at the base of the ambulacra, in the 

 greater size of the apical disc and anal opening, and, above all, in the absence of the broad, 

 crenulated band which surrounds the body of the primary spines above the milled ring. 



Locality and Stratigrapliical position. — The specimen figured was collected from the 

 Portland Sand. Those found in the Boulonnais were collected from the Portland Beds 

 of the Ealaise d'Alprecht, and at Ningle, by MM. Bouchard and Davidson, where it is 

 associated with a small elongated urchin, Ecliinohrissus Haimii, Wright, nov. sp. 



E. Species from the Marine Purbeck Beds. 



Hemicidaris Purbeckensis, Forbes. PI. V, fig. 4 a, b, c, d. 



Hemicidaris Purbeckensis. Forbes, Memoirs of the Geological Survey, Decade III, 



pi. 5. 



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



— — Desor, Synopsis des Echinides Fossiles, p. 5, pi. 11, fig. 5. 



Test sub-globose ; ambulacral areas narrow and nearly straight, with five pairs of small 

 semi-tubercles ; inter-am bulacral areas with eight small tubercles set on moderate-sized 

 bosses ; apical disc composed of unequal-sized genital plates ; primary spines sub-compressed, 

 with a broad band of fine longitudinal lines at the base, above the prominent milled riug. 



