THE TERTIARY FORMATION. 21 



inner plates obsolete ; contact-margin of the left valve grooved or rabbeted for the 

 reception of the slightly flanged edge of the opposite valve, and the dorsal edge pro- 

 vided with hinge-teeth, consisting of a longitudinal series of numerous minute tuber- 

 cles, with corresponding pits ; on the right valve the dorsal edge is tubercled or 

 toothed along its anterior and posterior thirds, and pitted in the central portion ; whilst 

 the left valve has fine teeth on the middle part, and pits on the rest of its dorsal edge : 

 surface of the valves punctated. In its hingement, Cyprideis closely resembles the 

 marine form Cytheridea. 



No. I. Cyprideis torosa, Jones. Plate II, figs. 1 a — I i; and Woodcut, fig. 2 



p. 16. 



Candona. torosa, Jones. Annals and Mag. Nat. Hist., 2d ser., vi, p. 27, t. 3, fig. 6. 



INCH. 



Length, -^L Recent: Gravesend. 



Pleistocene : Essex and Kent. 



Carapace oblong, varying in its proportions according to age. Valves convex ; 

 bearing for the most part a slightly raised marginal rim on all edges except the dorsal ; 

 rounded before and behind ; straight on the ventral, and more or less arched on the 

 dorsal border; right valve less oblong, smaller, and narrower posteriorly than the left 

 valve. The surface of the valves is marked with closely set angular pittings (fig. 1 i), 

 coarse in the older specimens, and is raised in adult specimens into 5 — 7 tubercles. 

 Young individuals in general have the surface almost even, or marked by a slight 

 transverse sulcus near the centre and just posterior to the lucid spots, which indicate 

 the position of the first-developed tubercle ; 3 — 4 tubercles on the posterior moiety of 

 the valve, and 1 — 3 smaller tubercles anteriorly, become apparent afterwards. 

 Occasionally well-developed tubercles are present in small, and even in young speci- 

 mens ; and, on the other hand, individuals reach a large size without being marked 

 with more than one (anterior) tubercle. 



The hinge is considerably developed (figs. 1 g, 1 It) ; the hinge-margin of the right 

 valve bearing anterior and posterior sets of " knurlings," which are received into 

 corresponding pittings on the hinge-margin of the opposite valve ; whilst the central 

 portion of the hinge has a smaller set of similar pits and teeth, but inversely arranged. 



Lucid spots small, four, arranged in a single transverse, or vertical, row immediately 

 behind the chief, or anterior, tubercle (System c). 



Dorsal aspect irregular-acute-oval ; anterior, somewhat hexagonal. 



This well-marked species occurs abundantly in the pleistocene sands of Grays, 

 Essex, and was found in equal profusion in deposits of similar age at Wear Farm, 



