INVERTEBRATE PALAEONTOLOGY. 319 



rather deeply channeled by the sudden inflection of the upper margins of the 

 volutions; aperture subovate, a little oblique, and on the inner side nearly 

 straight, equaling a little more than half the entire length; inner lip rather 

 thin ; umbilical chink small; surface appearing nearly smooth to the eye, but, 

 when examined by the aid of a lens, seen to be marked by minute, closely- 

 arranged lines of growth, crossed by equally fine, minutely-undulating, 

 revolving striae. 



Length of a medium-sized specimen, 0.65 inch; breadth, 0.43 inch; 

 length of aperture. 0.37 inch ; breadth of same, 0.20 inch. Angle of spire 

 nearly regular; divergence, 57°. 



This species will be at once distinguished from all of the otherwise 

 similar forms yet known from the Upper Missouri Cretaceous rocks, by its 

 more elevated spire, more slender general form, and more distinct revolving 

 stria?. In general appearance, it agrees more nearly with a Jurassic species 

 described by d'Orbigny under the name Natica calypso (Paleont. Fr. Terr. 

 Jur., II, pi. 292, figs. 9 and 10); from which, however, it differs in having 

 more convex whorls, and rather more distinct revolving striae. Among Creta- 

 ceous species, it may be compared with such forms as Natica Clementina and 

 N. Cassisiana, d'Orbigny, from the Cretaceous rocks of France; though it is 

 too evidently distinct from both to require an enumeration of the points of 

 difference. It closely resembles in form some species of the recent fresh- 

 water genera Viviparus aud Campelama (= MelanthoV); but, all its associates 

 being marine types, there is no probability that it belongs to either of those 

 groups. 



This is one of the fossil species already alluded to, in connection with 

 the remarks on the genus, that seem to show intermediate gradations between 

 Amaura and Amauropsis. By reference to the figures, it will be seen to stand, 

 as it were, almost exactly intermediate between those groups ; that is, in its 

 elevated and subturreted spire, and thinner inner lip, as well as in some other 

 respects, it agrees well with Amaura; though, in its proportionally larger 

 body-volution and aperture, deeply -channeled suture, and subperforate colu- 

 mella, it agrees more nearly with Amauropsis. If, as already intimated, it shall 

 be found necessary to unite those two groups under one name, then the shell 

 here described would have to stand under the name Amaura paludinceformis. 



Locality and position. — Sage Creek and Cheyenne River, Dakota Terri- 

 tory : in the upper part of the Fort Pierre group. 





