THE SHELL-Llx\lESTONE OF DUilHAM. 155 



form. B. Schaurothiaiia lias straighter dorsal and ventral mar- 

 gins than the present species; it is more produced anteriorly, 

 and its posterior extremity is angulate ; in lateral contour it dif- 

 fers in the acuteness of its extremities, and in the position of its 

 greatest diameter; and its dorsal overlap is decidedly greater. 



C. sub-reniformis has only occurred to me at Tunstall Hill, 

 where it is extremely rare. 



NOTE BY Ma. T. R. JONES ON CYTHERE SUB-RENIFORMIS, KIRKBT. 



Mr. Kirkby has already noticed his recognition of the Cy there- 

 like aspect of the carapace of this species, and its similitude to 

 the recent Cythere reniformis and C. albomaculata. I cannot think 

 that we have here the carapace of a Bairdta or even of a Cytlieri- 

 dels. In fact, the carapace reminds us very strongly of Cythere 

 albomaculata; and it is only its greater relative length, and some 

 slight differences of marginal outline, that support a specific dis- 

 tinction. The dorsal border appears to me to present a much 

 more definite hinge-line (occupying the middle two-fourths of 

 the dorsal border, pi. XI., fig. 23c) than Mr. Kirkby's figure shows. 



Cytheee amputata, Kirkby. PI. X., fig. 4.* 



Bairdia truncata, Kirkby. " Ann. Nat. His.," 3rd 

 ser., vol. ii., p. 423, pi. XI., fig. 4. 

 Length, l-25th of an inch; height, 1-GOth of an inch. 

 Carapace sub-rhomboidal, obliquely truncated at the ex- 

 tremities, straight on the back, and slightly inflated veu- 

 trally at the anterior half, gently swollen near the centre 

 of each valve, and compressed towards the borders ; veii- 

 tral margin somewhat sinuate, but almost straight ; antero- 

 dorsal region nearly straight, sloping to anterior extremity ; 

 postero-dorsal region slightly convex, sloping abruptly 

 downwards and forwards to the posterior extremity, and 

 parallel with the antero-dorsal border ; dorsal margin 

 straight, with a sub-angular projection at the anterior 

 hinge, near the anterior extremity, and terminating pos- 



* In this figure the margin of the carapace placed uppermost is the ventral, and the 

 lower one is the dorsal, 



