THE TERTIARY FORMATION. 47 



No. 1. Cytherideis trigonalis, Jones. Plate II, figs. 2 a — 2//. 



Cythere trigonalis, Jones. Annals and Mag. Nat. Hist., 2d ser., vi, p. 28, t. 3, fig. 5. 



INCH. 



Length, T ' T Recent : Peg well Bay, Rent. 



Pleistocene : Essex. 



Carapace obtusely triangular, convex, finely punctated with angular pittings (fig, 

 2 h) ; extremities obliquely rounded ; anterior portion much broader and somewhat 

 more depressed than the posterior ; ventral border nearly straight, dorsal angular ; 

 hinge-margin oblique and faintly developed, anterior hinge accompanied by slight 

 marginal teeth. Lucid spots (fig. 2y) of the System b, page 5. 



Dorsal aspect acute-oval. 



The outline-form of this species is not uncommon in the genera Cypris and Cythere, 

 and especially in the sub-genus Cytheridea. 



A unique specimen of this interesting form was obtained from the Pleistocene or 

 Post-pliocene formation at Clacton, and is identical with a recent form from the sand 

 of Pegwell Bay 1 (for which I am indebted to Mr. Pickering), except that the latter has 

 denticles, or short blunt spines, on the anterior and posterior margins, as is usual with 

 the genus Cythere. 



A smooth form, Var. l^vis, which occurs plentifully in the pleistocene sand at 

 Grays, differs from the Clacton specimen merely in the want of pittings. 



No. 2. Cytherideis tuberculata, spec. nov. Plate II, figs. 3 a — 2>f. 



INCH. 



Length, Vy Pleistocene : Red Crag, Essex. 



Pliocene : Crag, Suffolk. 



Carapace convex, sub-triangular, depressed anteriorly ; extremities rounded , 

 posterior end contracted ; ventral border slightly incurved ; dorsal border strongly 

 angular ; hinge-line occupying the posterior two thirds of the dorsal edge : surface of 

 valves ornamented with tubercles arranged loosely in about eight longitudinal rows, 

 with about twelve in the longest row. 



Dorsal aspect acute- oval ; end-view sub -ovate. 



Cythere {Cytherideis) tuberculata occurs in the Crag of Suffolk, and in the Red Crag 

 at Walton, Essex ; but appears to be rare. 



1 Probably this, like the specimens of Candona reptans and C. Candida, mentioned at p. 17, was not of 

 marine origin, but derived from the Stour River, which empties itself into the Bay. 



