Dr. T. Wright on the Cassidulidie of the Oolites. 205 



has given beautiful drawings of the Swiss variety, and Cotteau 

 indifferent ones of the larger and smaller varieties occurring in 

 France. 



Dysaster analis, Agass. 



Syn. Dysaster analis, Agass. Prodrome Mem. Neuch. vol. i. 

 Cully rites analis, Desmoulins, Tab. Synop. p. 368. no. 14. 

 Dysaster analis, Agass. Echiu. Suisse, i. p. 6. tab. 1. fig. 12-14; 



Gressly, Jur. Sol. p. 70 ; Desor, Monogr. des Dysaster, p. 10. 



tab. 2. fig. 8-10 ; Agassiz and Desor, Cat. raisonne* des Echi- 



nides, p. 32. 



Test suborbicular, inclining to oval, more or less depressed, round 

 and inflated anteriorly, slightly contracted posteriorly ; dorsal 

 surface convex, more declined from the vertex to the anus than 

 from the vertex to the anterior border ; anterior half forming 

 the segment of a larger circle than the posterior half; vertex 

 excentral ; apical disc nearly central, at which the single 

 ambulacrum and the anterolateral ambulacra converge ; the 

 postero-lateral ambulacra meet at the distance of y^ths of an 

 inch behind the anterolaterals ; anus nearly circular and pro- 

 minent, situated in the middle of the posterior border about 

 midway between the ambulacral arch and the basal angle ; 

 ventral surface flat, without undulations; mouth circular, 

 situated in a depression about the junction of the anterior 

 with the middle third of the ventral diameter ; sides convex, 

 suddenly declining from a slight median elevation. 



Height 1 inch, anteroposterior diameter 1 inch and T %ths, 

 transverse diameter 1 inch and y^ths. 



Description. — The nearly ovato-orbicular circumference and 

 depression of the dorsal surface, which is not uniformly convex, 

 with the flatness of the ventral surface, form a group of characters 

 by which this Dysaster is distinguished from its congeners. The 

 anterior half of the body is fuller and more developed, and forms 

 the segment of a larger circle than the posterior half. 



The single ambulacrum, which is lodged in a slight depression 

 on the anterior border, is wider than the others; the antero- 

 lateral ambulacra form graceful curves on the anterior sides of 

 the shell, and with the single ambulacrum converge at the apical 

 disc ; the postero-lateral ambulacra are situated very far back, 

 and occupy the posterior sixth of the antero-posterior diameter 

 of the test ; they converge at some distance above the anus 

 at a point situated y^ths of an inch behind the apical disc; a 

 \cry lam intermediate space is thereby formed for the develop- 

 ment of the posterior pair of interambulacra, the converging 

 point of the antero-latcral and postero-lateral ambulacra being 



