218 Br. T. Wright on the Cassidulidae of the Oolites. 



of the Oolites. It exhibits many changes of form in the different 

 strata, attaining however its greatest development in the upper 

 beds of the Inferior Oolite, from whence the specimen now be- 

 fore me was obtained. The ambulacra are broadly lanceolate, 

 the single area and the anterior pair are about the same length 

 and width, and the posterior pair are shorter and wider on the 

 dorsal surface, and longer than the others on the base. The 

 pores are placed widely apart, as much as T %ths of an inch at 

 the widest space ; they are united by transverse sulci formed 

 by the sutures of the small plates of the ambulacral area?; 

 the poriferous avenues are more depressed, and exhibit the 

 lines of union more distinctly than the plates occupying the 

 interporous space, which are upon a level with the interambu- 

 lacral plates, and like them are covered with tubercles; the 

 separation of the pores and their union by transverse sutures 

 extends to the margin of the test, at the basal angle they be- 

 come approximated, and from thence to the mouth they form 

 triple oblique pairs placed wider apart. The interambulacra are 

 of unequal width ; the anterior pair are the narrowest, the single 

 area is the widest, and the posterior pair are of intermediate 

 dimensions. In large specimens there is a slight depression down 

 the centre of each area in the line of the median suture ; the 

 anal valley is narrow above and expanded below ; it is of an incon- 

 siderable depth, has an acutely conical form with inclining walls, 

 and extends from the apical disc to the border. The anal opening 

 is situated about the middle of the valley, the area is slightly pro- 

 duced and deflected, and an inconsiderable concavity is formed 

 in its centre corresponding to the boundary of the valley. The 

 test is thick, and its surface is profusely covered with small 

 tubercles, so arranged that they form oblique lines ; the tubercles 

 are surrounded by circular depressions, and the intertubercular 

 surface is occupied by microscopic granules. The vertex is nearly 

 central, inclining in general to the posterior border, behind which 

 the apical disc is placed, formed of two pairs of perforated ovarial 

 plates, and a single imperforate plate which extends into the anal 

 valley, the centre of the disc being occupied by the spongy 

 madreporiform body ; the ocular plates are small, and have the 

 eye-holes near their margins. 



The base is flat, inclining to concave ; the mouth-opening is 

 excentral and pentagonal, being situated nearer the anterior than 

 the posterior border, and having its margin surrounded by five 

 prominent lobes ; the ambulacra form straight narrow valleys, 

 and the interambulacra are slightly convex, which occasions 

 gentle undulations on the basal surface, as in other Nucleolites ; 

 the tubercles are a little larger on the base than on the dorsal 

 surface. 



