252 Mr. T. R. Jones on North American 



Pennsylvania, belonging to the " Surgent group/' and of the 

 same age as the " Clinton gi-oup*/' is full of individuals of this 

 species. It exhibits numerous light-brown valves, of different 

 sizes, and in good preservation, showing smooth, non-punctate 

 surfaces, with the eye-spot and its escutcheon, and the muscle- 

 spot with its reticulated surface and delicate sinuous radii pass- 

 ing downwards and backwards (fig. 12). L. Pennsylvanica is 

 here accompanied by a few specimens of a minute Beyrichia 

 [B. Pennsylvanica t)> 



In some fragments of another greyish limestone of the " Sur- 

 gent group,'' we have numerous specimens, some well preserved, 

 of this species ; the valves are of a darker tint than in other 

 instances, and somewhat more convex (fig. 18). 



4. Leperditia ovata, spec. nov. PI. X. fig. 14. 



Length /q, breadth -f-Q- inch. 



In a specimen of the Blackriver-limestone (" Auroral group") 

 of Potter's Fort, Penn's Valley, Pennsylvania, — a bluish-grey 

 crystalline limestone, containing Spirifers and Encrinites, — occurs 

 a single right valve of a small Leperditia, black, smooth, unorna- 

 mented, having a nearly ovate outline and a convexity sufficient 

 to give a subglobose form to the closed carapace. The slightly 

 raised muscle-spot marks the centre of the valve ; but the eye- 

 spot is wanting. 



The want of angularity in this form, though it has a straight 

 hinge-line, its central muscle-spot, and absence of ocular tubercle 

 distinguish it from L. Canadensis, and ofi*er sufficient character- 

 istics to lead me to recognize it by the specific name of L. 

 ovata. 



The Cytherina fahulites oi Mr. Conrad t appears, from the brief 

 description given of it, to be somewhat aUied to the species before 

 us. Mr. Conrad's specimen is from the Trenton limestone of 

 Mineral Point, Wisconsin. 



5. Beyrichia Maccoyiana, Jones. PL X. fig. 15. 



Annals Nat. Hist. 2 ser. vol. xvi. p. 97. pi. 5. fig. 14. 



Numerous individual valves of this species, of a somewhat 

 larger size than the Scandinavian specimens, and in fine pre- 

 servation, occur in a flaky calcareous rock, almost wholly com- 



* For the classification of the Palaeozoic Rocks of North America, see 

 Prof. H. D. Rogers's Geological Map of the United States, in Keith John- 

 ston's * Physical Atlas ;' also Prof. James Hall's works on the Geology and 

 Palaeontology of the State of New York. 



t Philad. Acad. Nat. Sc. Proceed, vol. i. p. .'332. 



