300 CONTRIBUTIONS TO CANADIAN PALvEONTOLOfiY. 



very inequilateral. Valves strongly convex in the umbonal region, both 

 posteriorly and anteriorly, but slightly depressed in the centi-e below ■ 

 posterior area obliquely and abruptly compressed. 



Anterior side very short, angular above and rounding both rapidly and 

 -abruptly inward into the ventral margin below : posterior side much 

 longer and a little broader than the anterior, its extremity obliquely trun- 

 cated or .subtruncated above and narrowly rounded below cardinal border 

 nearly straight behind the beaks, in some specimens (as in fig. 3) nearly 

 parallel with the ventral margin, in others (as in fig. 2) ascending and 

 subalate posteriorly : ventral margin almost straight but faintly concave 

 in the centre and rounding upward very abruptly at each end : umbones 

 prominent and comparatively broad : beaks depressed, curved inward and 

 slightly forward. 



Surface markings consisting apparently of concentric and lamellose lines 

 of growth. Muscular impressions unknown : hinge dentition for the most 

 part unknown, though in the cast of the interior of the left valve repre- 

 sented by fig. 2, there are impressions of two of the thin, laminar and 

 elongated posterior teeth parallel to the hinge line. 



Maximum length of one of the most perfect specimens collected (the 

 right valve represented by fig. 3), ten millimetres, greatest height of the 

 same, inclusive of the beaks, five mm. and a quarter. 



A large mould of the exterior of the right valve, however, which is not 

 figured but which gives the only information available aVjout the surface 

 markings of the test, is a little over fourteen millimetres in length. 



Lake Winnipegosis, on the north-western shore, at Devils Point, in the 

 Upper Devonian (four single valves) ; and in the Stringocephalus zone at 

 Dawson Bay, on the south-east side, on a small island to the south-west 

 of Whiteaves Point (one right valve), and on the south-west side, at the 

 mouth of Steep Rock Rivei' (the left valve represented by fig. 2) : J. B. 

 Tyrrell, 1889. 



All the specimens from these localities, except the solitary mould of the 

 exterior of a right valve already referred to, are perfect and well pre- 

 served casts of the interior of the right or left valve. The .species is 

 perhaps most nearly related to, but probably distinct from, the J/. yitrvvK 

 of White and Whitfield,* from the yellow sandstone at Burlington, Iowa, 

 which Dr. White regards as the lowest member of the Lower Carbonifer- 

 ous in the Mississippi valley. 



Proo. Boston >Soo. Nat. Hist., 1882, vol. VIII, p. 299. 



