WH.TEAVES.] CRETACEOUS FOSSILS, NORTH WEST TERRITORY. 1*79 



posterior extremity is a very little the narrower of the two. Sujjerior 

 border nearly straight for some distance in front of and behind the 

 beaks, which are inconsisicuous, central, appressed and depressed; 

 ventral margin nearly straight or very faintly concave in the centre. 



Surface apparently marked only with concentric lines of growth. 

 Hinge dentition, muscular impressions and pallial line unknown. 



Approximate dimensions of the only specimen collected : maximum 

 height, twenty-three millimetres; greatest length, sixty-seven mm.; 

 thickness through the closed valves, about fourteen mm. 



Battle Eiver, Township 40, Eange 13, west of the 4th Principal 

 Meridian, 1885 : an imperfect and badly preserved left (?) valve. 



Martesia tumidiprons, Whiteaves. 

 Plate 25, figs. 1, la and 2. 

 Martesia tumidifnms, Whiteaves. 1887. Op. cit., p. 1.57 E. 



Shell rather large for the genus, very inequilateral, valves subglo- 

 boseor semiglobose and abruptly swollen in front, produced and rather 

 rapidly attenuated behind ; outline, as viewed from above, somewhat 

 pyriform. Greatest height, as measured in the centre, behind the 

 beaks, about equal to the maximum thickness through the closed val- 

 ves ; greatest height, as compared with the maximum length, about as 

 three to five. 



Lateral outline transversely subovate ; anterior side very short, its 

 outer margin broadly rounded but somewhat truncated inwardly below 

 the middle ; j)Osterior side much more elongated, narrowing gradually 

 at its upper margin and much more rapidly from below upward, its 

 narrow and conspicuously gaping extremity being apparently some- 

 what obliquely truncated, though the margins of the cast of the united 

 valves of the only specimen collected are both a little broken at this 

 point. Superior border rounding abruptly downward in front, and 

 neai-ly straight, but descending very gently behind: ventral margin 

 broadly rounded, most prominent a little behind the middle : umbones 

 swollen and prominent : beaks large, incurved and depressed, with a 

 slight forward inclination and placed very near the anterior end : 

 escutcheon broadly lanceolate and tolerably well defined. 



On the umbonal region of the left valve only, a small portion of the 

 test is preserved, and the outer surface of this is marked with concen- 

 tric and rather irregularly disposed, ridge-like folds, which are often 

 separated from each other by somewhat broader and rather deep con- 



