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



joining pieces of the shell, that it is only in weathered specimens, 

 or in those where the test has passed into the condition of calca- 

 reous spar, that we can distinguish the separate pieces of which 

 it is formed. The surface of the shell is covered with small 

 tubercles surrounded by circular depressions, between which 

 numerous microscopic granules are scattered; the spines are 

 unknown. 



Affinities and differences. — In its general outline I). bicordatus 

 resembles D. ringens, but is distinguished from it by the follow- 

 ing characters : in D. bicordatus the highest point of the back is 

 near the anterior third, whilst in D. ringens it is at the posterior 

 third ; in D. bicordatus the ventral surface is nearly uniformly 

 convex, in D. ringens it is very much undulated ; in D. bicordatus 

 the apical disc is situated near the anterior third of the dorsal sur- 

 face, whilst in D. ringens it is nearly central ; the single inter- 

 ambulacrum is not so much developed, the anal opening is 

 larger and higher up, and the anal valley is more rudimentary in 

 D. bicordatus than in D. ringens. By its height and dimensions 

 and the disposition of the ambulacra, and the high position of 

 the anus and rudimentary valley, D. bicordatus closely resembles 

 D. Robinaldinus. M. Cotteau however thinks it is distinct ; but 

 we have before us specimens from the Inferior Oolite so closely 

 resembling the figure given by him in his ' Etudes des Echinides/ 

 that we suspect Cotteau' s species to be a variety of D. bicordatus. 

 We have before remarked, that in our opinion a greater or less 

 elevation of the dorsal surface or a flattening of the borders of the 

 test do not per se constitute specific characters ; for this reason we 

 consider D. symmetricus as only an oval variety of D. bicordatus. 



Locality and stratigraphical range. — This species has been col- 

 lected with D. ringens by the officers of the Geological Survey 

 from the sands of the Inferior Oolite of Dorsetshire. I have 

 never had the good fortune to find it in the same stratum in 

 Gloucestershire ; it may however occur at Frocester and Wootton- 

 under-Edge, where the sands of the Inferior Oolite attain a great 

 development. 



History. — First figured and described by Desor in his ' Monogr. 

 des Dysaster/ afterwards by Cotteau under the name D. Robi- 

 naldinus in his 'Etudes des Echinides Fossiles/ and described 

 under the name D. symmetricus by Prof. M'Coy ; recorded as oc- 

 curring in abundance in the Inferior Oolites of Dorsetshire by the 

 officers of the Geological Survey. 



Dysaster ovalis, Agass. 



Syn. Dysaster ovalis, Agassiz, Cat. Syst. 3 ; Desor, Monogr. des 

 Dysaster, p. 15. tab. 3. fig. 21-23; Agassiz and Desor's Cata- 

 logue raisonne des Echinides, A. S. N. torn. viii. p. 32. 



