Dr. T. Wright on the Cassidulidie of the Oolites. 203 



Dysaster oralis, 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. 



Spatanyus ovalis, Parkinson, Org. Rem. vol. iii. tab. 3. fig. 3 ; Phil- 

 lips, Geol of Yorksh. vol. i. p. 127. tab. 4. fig. 23 ; Young and 

 Bird, Geol. of York. pi. 6. fig. 9. 



Var. A. Dysaster propinquus, Agass. Echin. Foss. Suisse, i. p. 2. tab. 1. 

 fig. 1-3 ; Desor, Monogr. des Dysaster, p. 14. tab. 3. fig. 24-26 ; 

 Agassiz and Desor's Catalogue raisonne des Echinides, p. 32. 



Var. B. Dysaster truncatus, Dubois, Voy. au Cauc. (Ser.Geol.) tab. 1. 

 fig. 1 ; Desor, Monogr. des Dysaster, p. 17. tab. 13, des Galerites, 

 fig. 8-1 1 ; Agassiz and Desor, Cat. raisonne' des Echinides, p. 32. 



Var. major. Height |§ths of an inch, antero-posterior dia- 

 meter 1 inch and y^ths, transverse diameter 1 inch and y^ths. 



Var. minor. Height J^ths of an inch, antero-posterior dia- 

 meter 1 inch and T %ths, transverse diameter £§ths of an inch. 



Description. — The general outline of this Dysaster is oval, the 

 anterior border however forms the segment of a larger circle 

 than the posterior border, which is produced and truncated ; the 

 upper surface is uniformly convex, the sides are tumid, and the 

 base is flat ; the ambulacral areas are relatively wide, the anterior 

 single ambulacrum and the antero-lateral ambulacral areas con- 

 verge at the apical disc, near the centre of the back ; the single 

 area descends in a depression which grooves the anterior border 

 of the test, and the base thereof as far as the mouth ; the antero- 

 lateral ambulacra are gently sinuous, and form graceful curves 

 over the sides of the shell, having their apices separated from 

 that of the single area by the anterior pair of ovarial plates ; the 

 posterolateral ambulacra are relatively wider than the antero- 

 laterals, they take a straight direction at the base, are slightly 

 sinuous on the sides, and converge at about the posterior third 

 of the back, at a point about midway between the apical disc and 

 the anal opening. The interambulacral areas are formed of large 

 plates, the single area is produced and sometimes truncated, and 

 the anal opening is situated at the superior part of the posterior 

 border between the point of convergence of the postero-lateral 

 ambulacra and the basal angle. 



The mouth-opening is exccntral and lodged in a depression 

 near the anterior border, about the anterior fourth of the base ; 

 the surface of the shell is covered with small tubercles, very uni- 

 form in size on the different parts thereof; the interambulacral 

 plates are convex inferiorly and concave superiorly, whilst the 

 ambulacral plates are united together by straight sutures \ the 



