200 UNITED STATES GEOLOGICAL SURVEY OF TEE TERRITORIES. 



marked by very obscure lines of growth, and traces of extremely fine, nearly 

 obsolete radiating stria?. Hinge and interior unknown. 



Length 0.69 inch ; height, 0.40 inch ; convexity, about 0.26 inch. 



I am still in much doubt in regard to the relations of this shell. In 

 first describing it, being inclined to believe that it might belong to some of 

 the sections of Tellina, we referred it provisionally to that genus. Subse- 

 quently, however, on noticing its very close similarity in form and general 

 appearance to the recent Abra intermedia, I was tempted to refer it, with a 

 mark of doubt, to that genus, in the Smithsonian Check-List. Un fortunately, 

 no specimen has yet been found in a condition to give any idea of the nature 

 of its hinge and internal characters. On picking t he rock from about the 

 cardinal margin, however, I found that it shows behind the beaks unmistakable 

 evidences of having had a decided external ligament, and from this fact it can 

 hardly belong to Abra, in which the ligament is only partly external. Hence, 

 I am now inclined to believe that it may possibly belong to Mr. Conrad's 

 genus Linear/a, as its radiating strise would seem to indicate, though it is 

 much more depressed and has much narrower extremities than any species 

 with which I am acquainted that is certainly known to belong to that genus. 

 In regard to its radiating stria?., however, I should remark that they are so 

 very faint that they can only be seen by the aid of a magnifier in a cross light ; 

 consequently, no attempt was made to show them on the figure. 



Locality and position. — Twenty miles below the mouth of Cannon-Bail 

 River, in the Fox Hills group, or No. 5 of the Upper Mo. Cretaceous series. 



Genus ARCOPAGELLA, Meek. 



Synon. — Arcopagella, Meek ( 1871), Hayden's Second Anu. Report U. S. Geol. Survey of the Territories, 308. 



Elym. — Arcopagia (dimin.). 



Type — Arcopagella mactroides, Meek. 



Shell equivalve, more or less nearly equilateral, longer than high, with 

 margins closed all around and smooth within. Hinge with two cardinal and 

 one anterior and one posterior lateral teeth in each valve. Left valve with 

 anterior cardinal tooth larger than the posterior, and trigonal in form, but 

 sometimes rather deeply emarginated below, placed directly under the beak ; 

 posterior cardinal tooth small, slender, and ranging obliquely backward and 

 downward close to the larger one, so as to leave only a slender pit between, 

 corresponding to another on the anterior side of the principal cardinal tooth, 



