THE CRETACEOUS EORMATION. 31 



No. 3. Cytherella Williamsoniana, noUs. Tab. VII, fig. 26, a — i. 







Var. 





YOUNG. 



ADULT. 



Granulosa. 





INCH. 



INCH. 



INCH. 





Length, J^ 



I 

 3 3 



1 



2 a 



Gault, Folkstone, and Leacon Hill. 



Height, ^^0 



^0 



1 







Chalk-marl, Dover. 



Thickness, 



^V 





Detritus, Charing. 



Chalk, South-East England. 



Carapace depressed, oblong ; when young, approaching to oval. Valves flat, with 

 a very shallow internal cavity ; having a smooth, convex ridge running close to the 

 anterior margin, and dying out on the superior and inferior margins ; a similar but 

 stronger ridge lies near the superior, inferior, and posterior borders of the valve, or 

 rather, two strongly-developed longitudinal ridges, one near the superior, and the 

 other near the inferior border, beginning on the anterior third of the valve, free of 

 the marginal ridge, and within it, run backwards to the posterior extremity of the 

 valve, and are there more or less perfectly connected by a cross ridge. Between the 

 anterior extremities of these ridges there are sometimes one or more short ridges or 

 club-shaped tubercles. In old shells, at the superior and inferior angles of the posterior 

 extremity of the valve, the ridge is developed into two large tubercles, having cor- 

 responding cavities on the inner surface of the valve. Extremities rounded ; posterior 

 extremitij slightly bearded. Dorsal edge slightly arched ; ventral somewhat incurved. 



Dorsal and ventral aspects narrow, irregular, elongate-oblong; anterior irregular, oblong. 



Named after Mr. Williamson, of Manchester, who has devoted much time and labour 

 to the investigation of the Entomostraca and other Microzoa, both recent and fossil. 



Variety Granulosa, fig. 26 L 



Chalk, Norwich. 



In this shell, which arrives at a comparatively large size, the ridges and tubercles, 

 especially the latter, are very strongly developed. The whole surface of the valve is 

 covered with granulations, coarser in the depressions than on the ridges. It occurs 

 rather plentifully in the Soft Chalk at Thorpe, near Norwich, and like others of the 

 Entomostraca of that locality has a reddish-brown colour. 



This depressed form of carapace is not very common. Besides the above- 

 mentioned species and its variety, there is one other species in the Maestricht Chalk, 

 described and figured by M. Bosquet as Cypridina auricularis (Op. cit. pi. iii, fig. 3), 

 and another in the Carboniferous Limestone of East Kilbride,^ Scotland. 



1 In the Hunterian Museum, among the fossil microscopic shells from East Kilbride. See note, p. 26. 



