Dr. T. Wright on the Cassidulidse of the Oolites. 295 



the shell, and with the single ambulacrum converge at the apical 

 disc ; the postero-lateral ambulacra are situated very far back, 

 and occupy the posterior sixth of the antero-posterior diameter 

 of the test; they converge at some distance above the anus 

 at a point situated T 7 oths of an inch behind the apical disc ; a 

 very large intermediate space is thereby formed for the develop- 

 ment of the posterior pair of interambulacra, the converging 

 point of the antero-lateral and postero-lateral ambulacra being 

 proportionally further removed in this species than in its conge- 

 ners. The interambulacral area? are of unequal width, the an- 

 terior pair are the narrowest, the posterior pair are the widest, 

 and the single interambulacrum is of intermediate dimensions; 

 the anterior pair are convex and prominent, and give the front of 

 the shell an inflated appearance; the posterior pair gradually 

 decline towards the anus and the lateral borders, and form a 

 broad saddle which extends across the back and sides of the 

 test. The single interambulacrum is small, and destitute of the 

 basal development so characteristic of the analogous area in 

 D. ring ens and D. bicordatus. 



The dorsal surface is not uniformly convex ; the anterior half 

 is rounder and more inflated than the posterior half ; the latter is 

 more depressed on the back and more contracted on the sides 

 than the anterior half, and forms the segment of a much smaller 

 circle than it ; the vertex is excentral and situated nearer the an- 

 terior than the posterior border, corresponding to about the apex 

 of the single ambulacrum ; the apical disc is nearly central, but 

 its structure is not preserved in the specimen before me ; accord- 

 ing to Agassiz, the holes in the four ovarial plates are very small, 

 and their position is not regular ; in general, the one correspond- 

 ing to the right posterior interambulacral area is behind that of 

 the left side. 



The anal opening is situated in the posterior border about mid- 

 way between the arch formed by the postero-lateral ambulacra 

 and the angle of the shell ; the aperture is large, prominent, and 

 of an elliptical or nearly circular form; the ventral surface is 

 flat, and in this respect presents a striking contrast to its con- 

 geners. In some of the best preserved specimens, the subanal 

 prominence formed by the interambulacrum, and which is so 

 prominent in D. ringens, is absent in D. arialis. The mouth- 

 opening, of a circular form, is situated in a slight depression 

 near the anterior border, at the junction of the anterior with the 

 middle third of the base. It has been remarked by Agassiz, who 

 had the advantage of examining a suite of perfect specimens of 

 this species from the Inferior Oolite of Switzerland, that the am- 

 bulacral plates diminish sensibly in depth when traced from the 

 - mouth to the points of convergence of the ambulacra ; as each 



