FROM THE GREAT OOLITE. 89 



comrae dans l'espece precedente. Les tubcrcules inter-arabulacraires diminuent sensible- 

 ment de grosseur a la face superieure."* The Hemicidaris aljrina, Agassiz, was collected 

 from the upper division of the Jurassic group, " Portlandien moycn (Kimmeridgien) de Gesne 

 et des Ormonds (Alpes vaudoises)," whilst Hemicidaris Wrightii was found in the lower 

 division of the same group, the Bradford Clay and Forest Marble. 



The test of this beautiful species is sub-globose ; the ambulacral areas are slightly 

 undulated, and of a medium size, rather depressed above; they are prominent and convex, 

 of an elongated conical form, and are thickly covered with small hemispherical granules, 

 without perforations or other sculpture (PI. V, fig. 2 d) ; the marginal rows are larger 

 and more regular ; between them are from four to six rows of smaller granules, closely set 

 together. At the base of the areas are four or six mammillated and perforated semi-tubercles 

 (PI. V, fig. 2 e), which are limited to this region. The pores are set obliquely in pairs, 

 with a smooth, elevated granule between each pair, which forms a moniliform sinuous 

 line, running between the pores (PL V, fig. 2 d). 



The inter-ambulacral areas are of moderate breadth, with two rows of primary tubercles, 

 five or six on each column. The bosses of the two central tubercles are large and promi- 

 nent (PL V, fig. 2/); those towards the anal and oral poles are smaller (fig. 2 a), and 

 they are all crenulated at their summits (fig. 2/); the tubercles are deeply perforated, 

 and supported on a short stem, the hemispherical head of the tubercle not exceeding in 

 diameter that of the stem (fig. 2/) ; the areolas around the basis are slightly channeled, 

 and they are all confluent ; those towards the anal pole have a circle of granules encircling 

 the areolas (fig. 2 a) ; the miliary zones are narrow, and covered with two rows of small, 

 smooth granules (fig. 2 b), similar in form and size to those occupying the ambulacral 

 areas. The apical disc (fig. 2 a and c) is very prominent ; the ovarial plates are large, 

 convex, and much granulated ; the two anterior pair are larger than the posterior pair, but 

 the right anterior plate is the largest (fig. 2 c) ; the genital holes are large, and near the 

 apices ; the ocular plates are of a proportionate size ; the spines are unknown. 



The mouth opening is of moderate size, the peristome between the lobes being deeply 

 notched and refiexed ; the pores are small, and separated by thick septa (fig. 2 d) ; they 

 are disposed in simple pairs nearly all the length of the poriferous zones, but are arranged 

 in double files around the border of the oral aperture, in such a manner as to occupy all 

 the free space in the ambulacral areas, resulting from the contraction of the inter-ambulacral 

 areas in the region of the mouth ; the surface of the septa developes prominent convex 

 elevations, which form a moniliform line separating the pores of each zone. 



Affini 'ties and differences. — Our specimen is smaller in size, but, with the differences 

 already pointed out, it much resembles Hemicidaris Alpha, Agassiz, from the Calcaire de 

 Saanen, in having the ambulacral areas closely crowded with small, uniform, and hemi- 



* 'Synopsis des lichinides Fossiles,' p. 54. 



12 



