94 CANADIAN MICRO-PALEONTOLOGY. 



thickness '55; but the greatest convexity in this latter form is above 

 (not below) the median line; and the concavity of the dorsal line is 

 more pronounced, the valve being swollen on each side at the ends of 

 the hinge-line, but more so behind than in front. 



Mr. XJlrich informed me, in 1888, that he had met with more than 

 one species of this kidney-shaped form in the Cincinnati beds, with a 

 smooth surface and slight ventral overlap, and that in his MS. of 1881, 

 unfortunately destroyed by accident, he had separated them as a dis- 

 tinct genus under the name Leioditia\ and there is no doubt that we 

 have here a specimen of the same kind, though slightly different, and 

 from a much later horizon. After further research, Mr. Ulrich sug- 

 gested, by letter, in 1890, that Cythere Cincinnati ensis, Meek, a variety 

 of the same, C. irregularis, Miller, and Leperditia radiata, Qlrich, 

 might probably be grouped under Barrande's genus Elpe, having much 

 in common with each other and with it, except that he found G. Cincin- 

 natiensis and Jj. radiata to have a subcentral pit and a radiate structure 

 of shell, which features, however, are of course, wanting in casts. 

 Although Barrande's Elpe pinguis^^ is more globose, and much larger 

 than these American specimens, yet it is quite feasible to suppose 

 that they may belong to the same genus ; the above mentioned forms 

 presenting the necessary gradations. 



Taking Elpe, then, as being probably the genus to which this little 

 fossil from Dawson Bay, Lake Winnipegosis, belongs, I propose to 

 call it E. Tyrrellii after the energetic member of the Canadian Geologi- 

 cal Survey who collected it. 



6. Leperditia (?) exigua, (nov). 



PI. 12, lig. 10. 



Length -^4 (hinge-line "56), height -48 mm. 



Small, subovate or ovate-oblong; rounded at the ends, but smaller 

 in front than behind; straight on the back, dorsal angles blunt; 

 smooth and convex. This little left valve has somewhat the same out- 

 line as the much larger right valve of L. Selwynii, fig. 3 a ; but it is 

 j-elatively fuller behind, and has no frontal slope. It may also be com- 

 pared with fig. 13 a, and even with pi. xiii, fig. 11a; but, though it 

 looks like a Leperditia, its small size and poor state of preservation 



a unique cast, from M. Barrande's stratum f. 2 of the stage F, of his Fauna III (Upper Silu- 

 rian) near Mnienian, Bohemia. " Syst. Silur. Boh^me," vol. I, Supplem. 1872, p. 510, pi. xxvi, 

 fig. 15. 



