184 



The most salient characteristics of this species are the pinched or 

 constricted anterior end, and the concavely arched margin of the valves 

 in the byssal region. The surface markings appear to be essentially 

 similar to those of M. concentrica. 



Oethodesma affine. (N. Sp.) 

 Plate 20, fig. 5. 



Shell elongated, nearly three times as long as high, and highest behind, 

 valves strongly compressed and very inequilateral. Anterior side very 

 short and narrowly rounded : posterior side much longer than the ante- 

 rior, increasing gradually and very slightly in height and depth poste- 

 riorly, though the cardinal margin behind the beaks and the posterior 

 half of the ventral margin are not far from parallel : posterior end more 

 broadly rounded than the anterior : ventral margin nearly straight or 

 slightly concave anteriorly and at the midlength, but faintly convex 

 posteriorly : cardinal border, behind the beaks, slightly ascending out- 

 vifard, almost straight but very slightly convex : beaks probably small, 

 depressed and appressed, evidently placed very near to the anterior end. 



Hinge dentition unknown, muscular impressions not distinctly defined. 



Maximum length of the only specimen collected, 59'4 mm.; greatest 

 height, 21 mm.; height at midlength, 19 mm. 



West side of Selkirk Island, > Lake Winnipeg, D. B. Dowling, 1891 : 

 a cast of the interior of both valves, in a loose piece of limestone. The 

 beaks are broken off, but in every other respect the cast is nearly perfect. 

 The specimen seems to indicate a species which is most nearly related to 

 0. curvatum of Hall and Whitfield,* from the Hudson River group of 

 Ohio, but -which differs therefrom in its iiatter valves, more obscure um- 

 bonal slopes, more evenly and less obliquely rounded posterior extremity, 

 and, more particularly, by its proportionately shorter and much broader 

 (or rather, higher) anterior side. 



Vanuxemia. (Species undeterminable.) 



Little Tamarack Island, D. B. Dowling, 1890 : an imperfect cast of the 

 interior of both valves of a specimen, which, Mr. Ulrioh thinks, repre- 

 sents an undetermined species of the V Hayniaiia section of the genus. 



Ctenodonta astaet^fobsiis, Salter. 



Gtenodonta astartwformis, Salter 1859. Geol. Surv. Canada, Can. Org. Rem., 



Dec. 1, p. 39, pi. 8, fig. 7. 

 Billings 18C3. Geol. Canada, p. 175, figs. 164, a-b. 



* Report of the Geological Survey of Ohio, vol. II. , pt. 2, p. 95, pi. 2, fig. 6. 



