GEOLOGICAL SURVEY OF. THE TERRITORIES. 309 



with the lateral tooth on that side; lateral teeth like those of the left 

 valve; the anterior one apparently fitting under that of the other 

 valve, and the posterior above that of the other. Muscular impressions 

 shallow; pallial impression with a moderate rounded sinus, directed 

 obliquely forward and upward. Ligament unknown, but believed to be 

 external. Surface without ornamentation. 



After searching in vain to find some defined group under which 1 

 could range this shell, with a reasonable degree of probability of its 

 properly belonging to the same, I have concluded to propose a new 

 genus for its reception. It seems to be related to Arcopagia^ but differs 

 in having anterior and posterior lateral teeth in both valves, as well as 

 in the form of the anterior cardinal tooth of the left valve and its 

 arrangement with relation to the other. At first I was inclined to think 

 it might find a place in Mr. Conrad's cretaceous genus Tellimera, but a 

 sketch of the hinge of his type received from him shows the latter to be 

 quite distinct, having a double anterior lateral tooth in the right valve, 

 and the cardinal teeth of the same consisting of one bifid very oblique 

 cardinal tooth, and immediately behind this a minute vertical second 

 cardinal. The hinge plate of his genus is also distinctly wider on the 

 anterior side than in the type under consideration. 



It is somewhat remarkable that the hinge structure of the type of 

 this genus is, in most respects, very similar to that of the group for 

 which I have proposed the name Orassatellina, while the whole aspect 

 of the two shells is so entirely different that I can scarcely doubt that 

 they really belong to two distinct families — Grassatellina probably 

 belonging to the Crassatelledce, and Arcopagclla, apparently to the Tel- 

 UnidcB. As closely as their hinges resemble, however, a critical compari- 

 son of the accompanying cuts will show them to present important dif- 

 ferences of details. 



It is probable this genus will be found to include other tellinoid 

 cretaceous shells, the hinges of which are yet unknown. 



ARCOPAGELLA MACTROIDES, (MEEK.) 

 A B 



A, hinge and interior of right valve. 



B, hinge and interior of left valve ; both nat. size. 



Shell longitudinally-subovate, width or height about two-thirds the 

 length, rather compressed or moderately convex ; pallial margin forming 

 a regular semi-elliptic curve from end to end; anterior margin narrowly 

 rounded, with the most prominent part near the middle ; posterior bor- 

 der more narrowly rounded than the anterior, particularly below, where 

 there seems to be the faintest possible tendency to form a flexure or 

 fold ; beaks moderately prominent, located very nearly centrally ; dor- 

 sal outline sloping almost equally before and behind the beaks, but with 

 the anterior slope slightly concave in outline above, and the posterior a 

 little convex ; muscular impressions faintly marked and rather narrow 

 subovate; pallial line with its rather shallow, broadly-rounded sinus di- 

 rected very obliquely forward and upward. Surface . apparently with 

 only fine lives of growth. 



Length of one of the larger specimens, 0.78 inch; height, 0.53 inch ; 

 convexity, about 0.2G inch. 



