72 HEMICIDARIS. 



basal expansion of the areas ; the pairs of pores (PI. Ill, fig. 2 d) are placed slightly 

 oblique, and the septa have raised eminences, which form a moniliform division between 

 the pores ; there are ten to eleven pores opposite each large tubercular plate, and at the 

 wide basal region of the zones they fall into triple oblique pairs. 



The inter-ambulacral areas are twice and a half the width of the ambulacral ; they had 

 from six to seven large plates in each column, the three or four inferior plates alone sup- 

 porting large primary tubercles ; the three upper plates are destitute of them, and, in lieu 

 thereof, have clusters of granules, similar in size to those of the ambulacra, developed on 

 their surface, and forming triangular, quadrangular, or pentagonal figures, according to the 

 number of granules in each group (PI. Ill, fig. 2 a) ; the large primary tubercles occupy 

 the entire surface of the rhomboidal plate (fig. 2 d) ; the boss is large and prominent 

 (fig. 2 e), its summit is deeply crenulated, and the tubercle is likewise large and deeply 

 perforated ; the areolas are smooth and gently inclined (fig. 2 <?), and around them is a 

 complete scrobicular circle, formed of from thirteen to fifteen round granules (fig. 2 d) ; 

 the base is flat (PI. Ill, fig. 2 b) ; the mouth opening is very large, thirteen twentieths of 

 an inch, that of the diameter of the test being one inch and two tenths ; the peristome is 

 not so deeply notched as in many other congeneric forms. 



The apical disc is concealed by hard adherent rock in the larger specimen, but is well 

 exposed in a smaller one (fig. 2 a) ; it consists (fig. 2/) of elongated heptagonal genital 

 plates, the one carrying the madreporiform body is the largest ; the oviductal holes are pierced 

 near the apices ; the ocular plates are distinctly heart-shaped, with a depression down the 

 centre of each, the orbit being formed by the notch ; the disc makes a slight prominence 

 on the upper part of the test, and the surface of the plates is destitute of any sculpture ; 

 the anal aperture is circular and central (fig. 2/). 



Affinities and differences. — This Urchin very much resembles Hemicidaris pustulosa, 

 Agassiz, but is distinguished from it in having narrower ambulacra, with larger, fewer, and 

 more prominent granules thereon ; the inter-ambulacra have likewise fewer primary tuber- 

 cles, and their areolas are surrounded with complete circles of well-spaced-out scrobicular 

 granules (fig. 2 d, e), whereas in Hemicidaris pustulosa the areolas are confluent. 



It is distinguished from Hemicidaris intermedia by the absence of tubercles from the 

 upper parts of the inter-ambulacra, and by the form and size of the granules covering the 

 ambulacra. It is so entirely distinct from all other congeneric forms, that it is impossible 

 to mistake it for either of them. 



Locality and Strati graphical position. — The two specimens figured in PL III, fig. 2, 

 were collected from the Inferior Oolite of Dundry Hill, associated with Diadema depressum, 

 Echinus germinans, and Polgcgphus Forbesii. In Mr. Lowe's cabinet there is a fine 

 specimen of this species, showing the base very well, which was collected from the Forest 

 Marble near Corsham, Wilts. 



