Dr. T. Wright on the Cassidulidse of the Oolites. 207 



to the adjoining portion of the test, and having the madrepori- 

 form body on the surface of the right anterior plate ; there are 

 three ocular plates at the summits of the ambulacra with small 

 eyeholes in depressions thereof; base concave and much undu- 

 lated ; mouth excentral, simple and pentagonal, situated near the 

 anterior border ; anus oblong or pyriform, marginal or submar- 

 ginal, situated at the posterior surface, and sometimes encircled 

 by the postero-lateral ambulacra. 



Dysaster ringens, Agassiz. 



Syn. Dysaster ringens, Agassiz, Echin. Foss. Suisse, Part 1. p. 5. 

 tab. 1. fig. 7-11 ; Agass. Cat. Syst. p. 3; Desor, Monograph 

 des Dysaster, p. 24. tab. 1. fig. 13-17; Cotteau, Etudes des 

 Echin. Foss. p. 46. tab. 2. fig. 10-13 ; Agassiz and Desor, Ca- 

 talog, raisonne, A. S. X. tome viii. p. 33 ; Forbes, Mem. Geol. 

 Surv. Decade in. pi. 9. 



Collyrites ringens, Desmonlins, 3rd Mem. sur les Echin. p. 368. 



Dysaster Eudesii, Agassiz, Cat. Svst. p. 3 ; Desor, Monogr. des Dvs- 

 aster, p. 23. t. 1. fig. 5-12.' 



Dysaster subringeas, M'Cov, Annals Xat. Hist. 2nd Series, vol. ii. 

 p. 415. 



Test suborbicular or subpentagonal, rounded anteriorly, rostrated 

 posteriorly ; dorsal surface convex and depressed ; sides tumid ; 

 vertex nearly central ; ambulacra widely disjoined, posterior 

 pair forming an arch above the anal opening ; anus pyriform, 

 marginal, and situated in a terminal sulcus; base concave, 

 much undulated; interambulacra tumid, the single posterior 

 area very prominent and much deflected ; mouth small, sub- 

 central and subpentagonal. 



Height y^ths of an inch, antero-posterior diameter 1 inch and 

 y'gth, transverse diameter 1 inch. As this Urchin presents very 

 variable proportions, we subjoin the following table of the di- 

 mensions of eight specimens from the Bridport Oolite by Pro- 

 fessor Forbes* in inches and twelfths : — 



Description. — The preceding table of dimensions shows how 

 much the general outline of D. ringens varies, being nearly orbi- 



* Memoirs Geol. Survey, Decade iii. pi. 9. 



