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



Locality and stratigraphicat range. — Dysaster ringens var. Eu- 

 ilvsii lias been collected in the sands of the Inferior Oolite in 

 Somerset and Dorset. Our specimens are from between Sher- 

 borne and Yeovil, Barton-Bradstock, Bridport, and Chideock 

 Hill. On the continent it has been found in the " Marnes Vesu- 

 liennes" of the Jura by Agassiz, in the ferruginous Oolite of 

 Normandy by Deslongchainps, and in the same stratum at Tour 

 du Pre by Cottcau. Prof. M'Coy states that it is not uncommon 

 in the Inferior Oolite of Leckhampton ; but this is a mistake, as 

 it is not within the experience of any of the local collectors, that 

 a Dysaster was ever found in that locality ; it is said however to 

 occur in the Cornbrash near Cirencester, but we have not seen the 

 specimens. 



History. — Dysaster ringens was recorded for the first time by 

 Agassiz in his ' Prodrome*/ and has been successively figured 

 and described in his c Echin. Foss./ by M. Desor in his ' Monogr. 

 des Dysaster/ by Prof. Forbes in his * Memoirs of the Geological 

 Survey/ by M. Cotteau in his l Etudes des Echinides/ and was 

 mentioned by Desmoulins under the name of Collyrites ringens. 



Dysaster bicordatus, Desor. 



Syn. Dysaster bicordatus, Desor, Monogr. des Dysaster, p. 9. tab. 2. 



fig. 1-4 ; Agassiz and Desor, Cat. raisonne des Echin., A. S. N. 



torn. viii. p. 31. 

 Dysaster symmetricus, M'Cov, Annals of Nat. Hist. vol. ii. p. 414, 



2nd Series. 

 Robinaldinas, Cotteau, Etudes des Echinides, p. 75. tab. 7. 



% 1, 5. 



Test thin, oval, anterior and posterior borders nearly uniform in 

 convexity ; sides tumid ; dorsal surface convex, sometimes flat- 

 tened ; vertex excentral, situated near the anterior third ; anus 

 supra-marginal, postero-lateral ambulacra forming an arch 

 over the anal opening ; base convex, without undulations ; 

 mouth-opening small, situated at about the junction of the 

 anterior with the middle third of the base. 



Height at vertex ^§ths of an inch, antero-posterior diameter 

 1 inch, transverse diameter T %ths of an inch. 



Descrijitioii. — The regular oval outline of this Dysaster forms 

 a contrast to the orbicular and subpentagonal figure of D. ringens 

 var. Eudesii ; the sides are tumid, the dorsal and basal surfaces 

 are smooth and convex, and the test has a uniformly gibbous 

 appearance ; the ambulacra arc all complete, passing from the 

 mouth to their terminations on the dorsal surface without inter- 



* M£m. de la Soc. d'llist. nat. dc Neuchatcl. 



