287 



Two other species of brachiopoda are represented in the collections 

 made by Mr. Tyrrell at some of the exposures on the north-east shore of 

 Lake Winnipegosis, or on the islands adjacent thereto. These are : — 

 three very imperfect specimens of a shell like a small Athyris ; and a few 

 casts of the interior, or moulds of the exterior, of the valves of a small 

 Camarotoechia. 



MOLLUSCA. 



Pelecypoda. 



Ptebinea occidentalis. (N. Sp.) 



Plate 28, figs. 1, 2 and 3. 



Shell small, always longer than high, and in some specimens conside- 

 rably elongated, the main body of each valve in the shorter specimens 

 being compressed convex, while that of the longer ones is strongly and 

 obliquely convex ; right valve almost if not quite as convex as the left ; 

 anterior wing more abruptly inflected than the posterior. 



Anterior side short and consisting usually of a well defined lobe-like 

 wing, that is straight above and rounded obliquely below ; posterior side 

 longer and much wider in the direction of its height than the anterior, 

 its lower and non-alate portion broadly rounded at the base, the extre- 

 mity of its upper and alate portion subtrunoate almost vertically in some 

 specimens, and obliquely so in others, the cardinal margin behind the 

 beaks being proportionately longer in some specimens than in others. 

 Hinge line straight and ascending from the anterior to the posterior end, 

 but apparently not very far from horizontal in some specimens ; umbones 

 moderately prominent, beaks small, appressed and placed near the ante- 

 rior end. Surface marked with numerous, regularly disposed and close- 

 set impressed lines. 



Hinge dentition and muscular impressions unknown. 



Abundant at an exposure on the north-west point of Ami Island, near 

 the north-east shore of Lake Winnipegosis, where most of the specimens 

 are compressed convex and moderately elongated. One of these is repre- 

 sented by figure 1, on Plate 28. Abundant also on the weathered 

 surfaces of small, thin slabs of limestone at the Grand Rapids of the 

 Saskatchewan River, below Old Portage, where the specimens are strong- 

 ly convex and much elongated. One of these is represented by figure 2 

 on the same Plate. At both of these localities, the specimens collected 

 are for the most part casts of the interior of the left valve, but a few have 

 either the whole or a part of the test preserved. The testiferous left 

 valve represented by figure 3 on Plate 28, is exposed on the weathered 

 surface of a small slab of limestone (apparently loose) from Swan Lake, at 



