WHiTEAVES.] CRETACEOUS FOSSILS, NORTH WEST TERRITORY. 165 



OsTREA Skidegatensis, Whiteaves. 



Ostrea Skidegatensis, "Whiteaves. 1884. Geol. and Nat. Hist. Surv. Can., Mes. 

 Foss., vol. 1 p. 243, fig. 12. 



Two caste of the interior of the shell of an Ostrea which is probably 

 referable to this species. 



ExoGTRA. Species undeterminable. 



Four casts of what appear to be the convex valves of a small myti- 

 loid or subtriangular and somewhat arcuate Exogyra, which the writer 

 has not been able to identify with any known species but which are 

 not in a satisfactory condition for description or illustration. 



Lima perobliqua. (N. Sp.) 

 Plate 22, figs. 3 and 3 a. 



Shell of medium size, strongly compressed, very inequilateral and 

 broader than long ; marginal outline obliquely semioval. Anterior side 

 or buccal region nearly straight, but very slightly concave in the mid- 

 dle, its margins being deeply but narrowly inflected ; posterior side or 

 anal region broadly rounded, but truncated or subtruncated in the car- 

 dinal region ; pallial border narrowly rounded. Beaks moderately 

 pi'ominent, anterior, terminal, the posterior umbonal slope forming 

 nearly a right angle with the anterior. Bars and cardinal area un- 

 known. The only portion of the test that happens to be preserved in 

 either of the two specimens collected, is a small piece round the anal 

 margin of the less perfect of the two. On this part of the shell the 

 surface ornamentation appears to consist of very fine and delicate 

 radiating strife or impressed lines, which are much narrower than the 

 flattened spaces between them, and of concentric lines of growth. 



Dimensions of the most perfect specimen collected : greatest length, 

 forty-one millimetres ; maximum breadth of the same, fifty-seven mm. 

 A not very well preserved but nearly perfect cast of the interior of a 

 left valve and a portion of another. 



Although its surface markings are very imperfectly exhibited, the 

 lateral outline of this shell seems to be very different to that of any 

 other species of Lima that has so far been described and figured as oc- 

 curring in the Cretaceous rocks of North America. 



