.'504 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



teristic internal thickening and denticulation of outer lip, at intervals, are well 

 defined. 



It is possible that a critical study of more extensive collections may 

 bring to light reasons warranting the arrangement of this group of shells as a 

 section of the genus Tritonium ; but, with our present material for compari- 

 son, I cannot believe that they can be properly included in that group, as 

 typified by the recent Murex tritonis, Linnaeus. 



In several respects, the type of this genus resembles the recent Argo- 

 buccinnum Orcgonense; and at one time I supposed it to be so closely related 

 to that shell that the two forms might be congeneric. A more critical com- 

 parison, however, with specimens of the west-coast species, in the collection 

 of my friend Mr. Dall, leads me to believe that they do not belong to the 

 same genus. At any rate, the A. Oregonense shows no traces of the inter- 

 nal crenate varices seen in our type: while it always has a kind of tooth or 

 callus near the top of the innei side of the aperture, and a strong twist and 

 flexure of the beak, that do not occur in the type under consideration. 



So far as yet certainly known, this genus would seem to be confined to 

 the Cretaceous rocks. 



Xrachytritoii 'vincnl u m , H. & M. (sp.). 



Plate 19, ligs. 7, a, 6, c, d. 



Buccinum ? vinculum, Hall and Meek (1856), Mem. Am. Acad. Arts and Sci., V (n. s.), 390, pi. iii, fig. 5, a, h. 

 Fusus vinculum, Meek and Hayden (1860), Proceed. Acad. Nat. Sci. Philad., XII, 185. 

 Tvachytvilou vinculum. Meek (1864), Smithsonian Check-List N. Am. Cret. Fossils, ii2 and :!/. 



Shell thin, fusiform; spire conical, equaling about four-fifths the length 

 of the aperture and canal ; volutions about six, convex, last one slightly 

 ventricose, and tapering rather rapidly into the moderately-produced beak 

 below ; suture distinct ; surface ornamented by small vertical folds, or costs?, 

 crossed by little band-like revolving ridges that are as strongly elevated 

 between, as upon, the vertical folds; aperture oval, approaching obovate, 

 rather obtusely angular above, and tapering into the canal below; outer lip 

 thin, excepting where it becomes thickened and denticulate within to form 

 the internal varices, broadly sinuous above, and slightly more prominent in 

 outline below; inner lip well developed, and a little spread on the body- 

 volution above ; columella moderately arcuate. 



Length of the most nearly perfect specimen seen, 1.35 inches; breadth, 



