from the Fossiliferous Limestone of Durham. 329 



describe it provisionally as one that is new, to which, however, 

 I forbear adding a name, which may be attached when its spe- 

 cific identity is more firmly established. 



7. Bairdia reniformis, n. sp. PI. X. figs. 13 & IS a. 



Length ^j inch ; height y'j inch. 



Carapace subreniform, flatly convex, obtuse marginally, smooth. 

 Dorsal margin almost straight centrally, but slightly inclined 

 towards the anterior extremity ; anterior slope short, convex ; 

 posterior slope abrupt, convex, falling two-thirds of height. Ven- 

 tral margin sinuous, convex towards extremities, with a rather 

 deep sinus anteriorly situate. Anterior extremity rounded. Pos- 

 terior extremity obtusely pointed. Lateral contour sublenticular, 

 compressed, convex towards extremities, more so posteriorly than 

 anteriorly ; greatest diameter central, rather more than one-fourth 

 of length. Hinge with left dorsal margin moderately and regu- 

 larly overlapping the right. 



B. reniformis has some resemblance to B. Reussiana, and also 

 to B. Schaurothiana. It may be distinguished from the former 

 by its more Cythere-\ike form, by its more depressed dorsal 

 margin, its convex posterior slope, its unrostrated posterior, and 

 the greater width of the anterior extremity ; it also differs in 

 lateral contour, its extremities being convex and comparatively 

 blunt. B. Schaurothiana has 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 

 differs in the acuteness of its extremities and in the position of 

 its greatest diameter. 



At first I was inclined to refer this species to Cy there*, its 

 general resemblance to that genus causing me to consider it to 

 belong to it rather than to Bairdia ; but I found its mode of 

 hingement like that of Bairdia, so that there can be little doubt 

 of its belonging to the same group. 



Bairdia Schaurothiana, n. sp. PI. X. figs. 14 & 14 #. 



Length T * F inch; height J$ inch. 



Carapace subhexagonal ; dorsal half convex, ventral less so, 

 protuberant about one-third from posterior extremity. Dor- 

 sal margin straight centrally ; anterior slope convex, gradually 

 descending rather more than one-third of height ; posterior slope 

 very abrupt, descending two-thirds of height in a right line. 

 Ventral margin straight, rising abruptly to meet dorsal margin 

 posteriorly, and very convex anteriorly. Anterior extremity 

 regularly rounded, prominent. Posterior extremity angulate, 



* To the recent forms C. reniformis and C albo-maculata, for instance. 



