Palaozoic Bivalved Entomostraca. 247 



Sheikas Island, Cornwall, on the St. Lawrence*; with Atrypa 

 plena (Chazy). 



Cornwall ; in the Trenton limestonef- 

 Lancaster; in the Black River limestone J. 

 Winchester; in the Trenton limestone §. 



Beyrichia Logani and Leperditia Canadensis occur together in 

 immense numbers, forming indeed a considerable portion of the 

 rock — a limestone, 1 foot 10 inches thick — in which they are 

 chiefly found. I believe that the former is not the young of the 

 latter (although, perhaps, the differences of shape and structure 

 are not greater than such as we find to occur between the young 

 and adult forms of recent Entomostraca and other Crustacea), 

 because, where the allied Beyrichics, such as B,strangulata\\f B. 

 mundula, and B, simplex y occur, even in equal numbers, in the 

 rocks of other localities, the Leperditim are not found with them ; 

 the latter also occurring unaccompanied by these Beyrichice ; and 

 L. Canadensis itself being found isolated in Grande Isle. The 

 ^ close resemblance in outline of some specimens of B. Logani 

 (var. B. leper ditioides, fig. 10) to the Leper ditice is, I believe, 

 merely a mimetic resemblance of outline, such as we find taking 

 place among many groups, both of the lower and the higher 

 animals. 



3. Leperditia Anna^j spec. nov. PI. IX. fig. 18. 



Length ^, breadth | inch. 



Small, convex; ovate-oblong, somewhat narrower in front 

 than behind; the ventral curve nearly uniform; hinge-line 

 straight ; dorsal angles slightly truncate. Surface of valves most 

 convex at the posterior third ; smooth, thickly punctate, each of 

 the little shallow circular pits having a minute central tubercle. 

 Eye-spot distinct and raised. 



Several valves of this neatly-pitted Leperditia are present in a 

 small hand-specimen of a hard, dark-coloured, concretionary 

 limestone, under the zone of Atrypa plena, and belonging to the 

 Calciferous Sandrock, from " immediately behind the village of 

 St. Ann's**," at the confluence of the Ottawa and St. Lawrence. 

 This is probably the oldest known species of the genus. 



* Geol. Surv. Canada Report, 1851-52, p. 70. t Ibid. 



X Ibid. p. 71. § Ibid. p. 72. 



II Beyrichia strangulata takes on a variety of forms (see Annals Nat. 

 Hist. vol. xvi. pi. 6. figs. 18-22) analogous to those of B. Logani. 



IF Referred to in Quart. Journ. Geol. Soc. vol. viii. p. 204. 



** Quart. Journ. Geol. Soc. loc. cit. ; and Geol. Surv. Canada, Report, 

 1851-52, p. 16. 



