Dr. T. Wright on the Cassidulidie of the Oolites. 199 



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-shapcd plates, which articulate before with the anterior 

 ovarials, laterally with the apices of the an tero- 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 Dysaster. 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 M. Moreau, 

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

 mens of D. ringens presenting various degrees of tumidity and 

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

 <r nidations 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 Mnall 

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

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

 the officers of the Geological Survey. 



* Etudes des Echinides Foss. p. 48. 

 t Mem. Gcol. Surv. Decade iii. pi. 9. 



