CYTHEREIS. 33 



toothed. Surface reticulated, ornamented with two prominent ridges, the dorsal 

 and most striking of which, strong, fenestrated, and somewhat convex, partly 

 obscures the hinge-line, and cvirves forwards and downwards below the front 

 hinge. The ventral ridge is not so strong ; both are sharply angular posteriorly. 



The only marked difference between our specimen and that figured by Dr. 

 Speyer is — that the dorsal ridge in the former is much better developed, being 

 higher, thinner, fenestrate, more delicate, and ending posteriorly in a much 

 sharper angle. 



A single valve from the White Crag. (British Museum.) 



4. Cythbbeis Peestwichiana, Jones ^ Sherhorn. Plate II, figs. 13, 14 a, h. 



CxTHEEEis Peestwichiana, Jones & Sherhorn. Greol. Mag., 1887, p. 454, pi. xi, 



figs. 11 a, b. 



A very small neat Oythereis, with well-developed marginal rim in front, which, 

 passing along the ventral region, gradually rises higher, and ends in a sharp 

 rectangle. A similar, but weaker, ridge follows the dorsal edge. Both are more or 

 less crenulated. There is a central boss, and a short ridge behind it, ending, like 

 the others, at the sudden posterior slope, which terminates in a narrow, produced, 

 flat, and toothed edge. The surface of the valve is somewhat depressed, and is 

 covered with a distinct lace-like reticulation. Edge-view subsagittate. 



This form is clearly related to that figured in the 'Monogr. Cretaceous 

 Bntom.,' 1849, pi. v, fig. 13 b, which we propose to remove from G. ornatissima 

 (' Greol. Mag.,' 1870, p. 75). We now have closely allied forms from the Chalk 

 of other localities in the British Islands, and the distinctness of this new species, 

 named after Prof. Prestwich, D.C.L., F.R.S., becomes more and more apparent. 



Two valves from the London Clay of Whitecliff" Bay, Isle of Wight. Collected 

 by Mr. C. D. Sherborn, F.G.S. (British Museum.) 



5. CxTHEKEis ARANEA, Jones Sj' SJierbom. Plate II, figs. 15 a, h. 



Cytheeeis aeanea, Jones & Sherhorn. Geol. Mag., 1887, p. 453 pi. xi, figs. 10 a, h. 



Oblong, with the front margin broader and rounder than the hinder, both 

 more or less denticulate. The surface ornamented with a delicate raised network 

 of irregular meshes, which extends over the flat ventral area. Two ridges, over 

 which the network is traceable, are present. One, shorter than the other, occupies 

 the median line from about the centi-e to the edge of the posterior slope, which makes 

 a strong depression at the hind margin. The other and longer ridge commences in 

 a curve inside the front margin, rises as it borders the ventral region, and dies out 



5 



