60 CONTRIBUTIONS TO CANADIAN PAL.5;0NT0L0GT. 



nearh' purallel for the greater part of their length. Margins of both 

 extremities evenly rounded in some specimens, but in others the pos- 

 terior end is bluntly pointed just below the middle. Superior border 

 descending obli(|nely, convexly and abruptly in front of the beaks, 

 nearly straight and horizontal, but slightly convex behind them : ven- 

 tral margin also nearly straight except at the immediate extremities, — • 

 apparently never concavelj' arcuate near the centre ; sides of the valves 

 also never concave near the raidlength below. Beaks very small, 

 depressed, ill defined and apjoroximated, placed very near the anterior 

 margin but not Cjuite terminal. 



Surface marked with the usual concentric lines of growth. Hinge 

 dentition unknown. 



Dimensions of the most perfect s]jecimens collected : length, one 

 hundred and fifteen millimetres, or a little m(]re than four inches and a 

 half : height of the same, fifty-one mm. In this individual, which is a 

 little distorted and twisted to one side, the valves are partially open, so 

 .that the exact thickness through them is diflicult to ascertain, but in 

 another specimen which appears to belong to the same species and 

 whose valves are closed, the maximum height is fifty millimetres, and 

 the greatest thickness of both united is about thirty. 



The species attains to a still larger size than this, for a cast of the 

 interior of the valves tVom another locality measures fully one hundred 

 and thirtj'-tive millimetres in length, by sixty-five in hein'ht. 



Milk Eiver Eidge, E. G. MoConnell, 1882 : one very large and nearly 

 perfect cast of the interior of both valves. Eed Deer Eiver, Township 

 21, Eange 12, west of 4th Principal Meridian, E. G. McConnell, 1883 : 

 one perfect specimen with the whole of the tost preserved, three imper- 

 fect but well preserved specimens, and one cast of the interior. 



Some casts of a large Unio, which are probably also referable to this 

 species, were collected by G. M. Dawson in 18^74, six miles west of the 

 fir-t branch of the Milk Eiver, while attached to H. M. North Ameri- 

 can Boundary Commission ; also, in 1881, on the Bow Eiver, ten miles 

 below G-rassy Island,— and by Jlr. McConnell, in 1883, on the South 

 Saskatchewan, eight miles above tlie mouth of the Eed Deer Eiver. 



So few j.erfect specimens of this shell have yet been obtained that its 

 specific relations are by no means clear. The specific name suggested 

 for It, which must be regarded as purely provisional, is inten.led to 

 convey the idea that its characters are of a very ordinary kind and 

 ones that are shared by it in common with many fossil and recent 

 species of Unio. It may be only an unusually large variety of Unio 

 Dance, but appears to be proportionately broader in the direction of its 

 height than that shell is, its ventral margin is not distinctly arcuate 

 If at all, and its flanks are never shallowly concave near the midleno-th 



