Dr. T. Wright on the Cassidulida? of the Oolites. 209 



lateral ambulacra; on the surface of the right plate is placed the 

 madreporiform body ; behind and between them is a small dia- 

 mond-shaped plate occupying the median line, its anterior angle 

 uniting with the apex of the single ambulacrum, and its poste- 

 rior border with the anterior ovarials ; behind these are two rhom- 

 boidal-shaped plates, which articulate before with the anterior 

 ovarials, laterally with the apices of the antero-lateral ambulacra, 

 and behind with the posterior pair of ovarial plates ; near the 

 points of junction of these plates with the ambulacra, the small 

 eyeholes are situated ; behind the rhomboidal ocular plates, the 

 small oblong posterior ovarial plates are situated. I can detect 

 no ocular plates at the summits of the posterior ambulacra ; a 

 fact, which in some measure serves to account for the concentra- 

 tion of the formative power on the three anterior ocular plates 

 which exhibit such a disproportionate development in the apical 

 disc of Dysasler. As this structure has not been accurately de- 

 scribed by former observers, I have taken advantage of the cir- 

 cumstance of having before me a specimen most favourable for 

 this purpose, and which I have carefully examined with the mi- 

 croscope under an inch object-glass. 



The mouth is more or less subcentral, and lodged in a conca- 

 vity ; it is of a pentagonal form, and is about one-eighth of the 

 length of the shell. The surface of the test is covered with small 

 tubercles having punctated summits, and surrounded by a cir- 

 cular depression ; they are larger on the ventral than on the 

 dorsal surface, but are microscopic on both, and the intermediate 

 surface of the plates is minutely granulated. 



Affinities and differences. — Many of our specimens of this 

 Urchin agree with the figures of D. Eudesii in M. Desor's mo- 

 nograph, whilst others have the depressed dorsal surface and 

 angular outline of D. ringens, and as we have a series of inter- 

 mediate forms connecting the extremes, it is probable that the 

 former may only be a variety of the latter species. On this sub- 

 ject M. Cotteau* observes, that he collected with If. Moreau, 

 from the " Oolite ferrugineuse " of Tour du Pre, a suite of sjreji- 

 mens of D. ringens presenting various degrees of tumidity and 

 more or less circularity of outline, and among which were all the 

 gradations conducting to D. Eudesii, from which he concluded 

 that the individual figured in his monograph, and which may be 

 taken as a fair representation of many of our specimens, is a small 

 and more elongated variety of D. ringens. This conclusion, ac- 

 cording to Prof. Forbesf, is in accordance with the experience of 

 the officers of the Geological Survey. 



Locality and stratigrajjhical range. — Dysaster ringens var. Eu~ 



* Etudes des Echinides Fuss. p. 48. 

 t Mem. Gt-ol. Surv. Decade iii. pi. 9. 

 Ann. $ Mag. N. Hist. Ser. 2. Vol. ix. 14 



