jMi-. T. R. Jones on some species of Lcperditia. 91 



tcrior third ; extremities obliquely rounded ; ventral margin 

 gently curved ; convexity greatest on the median line. 



Extremities of right valve depressed and margined with a nar- 

 row tlattened rim ; ventral margin incurved, and but slightly 

 overlapping the opposite valve. 



Left valve thickened along the dorsal border, especially pos- 

 teriorly, where it presents a smooth, oval, ridge-hke protu- 

 berance, short and depressed in young specimens, but prominent 

 and occupying even more than the middle third of the dorsal 

 edge in old individuals. The protuberance is separated from 

 the anterior tubercle by an oblique furrow. This dorsal protu- 

 berance is not present in the right valve at any age. The ex- 

 cessive thickening of the postero-dorsal portion of the left valve 

 is also present in M. Kouault's Bretagne species * ; and a ten- 

 dency to this condition is indicated in L. Arctica, and very 

 slightly in L. Bulthica. 



The rest of the border of the left valve is suddenly depressed 

 into a narrow, flat, and trenchant marginal rim ; the inverted 

 ventral edge is narrow. 



Anterior tubercle well marked; central tubercle indistinct. 

 Surface of valves smooth, shining, finely pitted, and of a rich 

 brown colour. 



L. (jihbera occurs in numbers in the Upper Silurian lime- 

 stone of Beechey Island, in company with numbers of small 

 bivalved Entomostraca, probably Beijrichia, among which are 

 two varieties of B. Kloedeni. Specimens of the limestone have 

 been brought to England by Capt. Sir E. Belcher, and are now 

 in the Museum of Pi-actical Geology. 



6. Leperditia marginata, Keyscrling, sp. PI. VII. figs. 11-14 

 & 15. 



Cypridina marginata, Keyscrling, Wissenschaftliche Beobach- 

 tungen auf eincr Reise in das Petschora-Laud (Geognostische 

 Beobachtungen), 1846, p. 288. pi. 11. fig. 16. 



Cypridina Balthica, Eichwald (non Cytherina Baltliica, Hisin- 

 ger). Bullet. Imp. Soc. Nat. Moscou, 1854, No. 1, p. 99. pi. 2. 

 fig. 6. 



A specimen of whitish (dolomitic) Upper Silurian limestone, 

 containing numerous specimens of a small Leperditia (chiefly 

 on a divisional plane of the rock), from the village of Saretche, 

 in the Government of St. Petersburgh, has been kindly presented 

 to me by Mr. T. Davidson, who received it from Prof. Kutorga 

 of St. Petersburgh. It was labelled " Cypridina marginata, Key- 



* Bullet. Soc. Geol. France, 2 ser. vol. viii. p. .377. tig- 1- 



