Dr. T. Wright on the Cassidulidae of the Oolites. 201 



ruption, and both on the dorsal and ventral surfaces being on a 

 level with the interambulacral areac ; the three anterior ambu- 

 lacra converge about the anterior third of the back ; the apex of 

 the single anterior area is separated from those of the antero- 

 laterals by the anterior pair of ovarial plates; the single ambu- 

 lacrum is the narrowest, and the postero-lateral ambulacra are 

 the widest; the anterior border is slightly flattened, and in the 

 cent re of this space the single ambulacrum passes in a straight 

 line from the vertex to the mouth ; at the apex of this area there 

 is a small prominent subtriangular eminence which forms the 

 highest point of the test, so that in this species the vertex is 

 situated at the anterior third of the back, whilst in D. ringens it 

 is in the posterior third; the antero-lateral ambulacra curve 

 gently upwards, backwards and forwards from the mouth to the 

 apical disc, forming thereby a gently undulated line; the pos- 

 tero-lateral ambulacra take a long sinuous course from the 

 mouth, passing backwards, outwards and upwards over the pos- 

 terior border, and converging near the median line above the 

 anal opening, over which they form an arch ; the apices of the 

 postero-lateral ambulacra converge on the back at the distance 

 of /^ths of an inch behind the antero-lateral ambulacra; the 

 pores are placed in oblique pairs, and are very distinct on the 

 sides and back, but on the ventral surface they are small and 

 very indistinct ; in the specimens before me the interambulacral 

 plates are large and bent, each forming a double inclined plane ; 

 the area? are uniformly smooth and gibbous, the single interam- 

 bulacrum is obliquely bevelled and slightly flattened, and at the 

 extreme upper part of this area, nearly on a level with the dorsal 

 surface, the anus is lodged ; the opening has a pyriform shape 

 with the apex directed upwards, from its lateral walls two obtuse 

 ridges pass downwards and outwards ; the basal portion of this 

 area is a little more gibbous and produced than the other area. 

 The ventral surface is convex with scarcely any undulation, an- 

 teriorly there is a slight concavity, and posteriorly an increased 

 convexity occasioned by the gibbosity of the single interambu- 

 lacrum ; the mouth is lodged in a slight depression, the opening 

 is small and subpentagonal, and is situated about the anterior 

 fourth part of the antero-postcrior diameter of the test ; the exact 

 relative situation of this aperture appears to vary in the different 

 individual! I have measured. The apical disc is situated behind 

 the subtriangular apical eniiiunee, and therefore occupies the 

 anterior third of the back; it has a lengthened rhomboidal tigure 

 and is formed very much like the disc in D. ringens, of which 

 we have already given a detailed description ; the four perforated 

 ovarial plates and the three large curiously implanted ocular 

 plates are seen very distinctly in the specimen before me; the 



