INVERTEBRATE PALAEONTOLOGY. 539 



As the name fragilis, under which we firsl indicated tliis shell, had been 

 previously applied I > \ Dr. Dunker to a recent Planorbis, il became necessary 

 to give our species another name, consequently we changed it to P.plano- 

 cnnvexus in the paper cited at the head el' this description. 



The only specimen of this species yet seen, consists mamly of a cast 

 of the interior, with portions of the thin shell attached. It has evidently 

 suffered some accidental flattening and erosion above, which has also 

 perhaps given greater angularity to its upper peripheral margin than is 

 natural; but it doubtless had these characters naturally more or less marked. 



Locality and position. — Fort Berthold, Dakota, on the Missouri River ; 

 from the Fort Union division of the Brackish- and Fresh-water Lignite 

 series. 



Planorbis i B a t li ,v o en plialus) amplexus, M. & H. 



Plate i-2, li-s. Hi, „,h,b,(l,r. 



Planorbis amplexus, Meek and Hayden (1S57), Proceed. Acad. Nat. Sci. Philad., IX, 21. 

 Helix {Polygyra) amplexus, Meek and Hayden (lsij(l), ib., 185. 



Shell planorbicular ; spire flat, or very nearly so; umbilicus deep, con- 

 ical, and about as wide as the under side of the last turn ; volutions about 

 six, closely coiled, and increasing very gradually in size, very narrow and a 

 little convex above, more broadly rounded on the outer side, and convex and 

 broad below, where they distinctly overlap each other, and become obtusely 

 subangular around the umbilicus; suture well defined; aperture and lip 

 unknown ; transverse section of the whorls obliquely semi-lunar, very narrow, 

 and deeply arcuate on the inner side; surface obscurely striated. 



Greatest breadth, 0.43 inch ; height, 0.15 inch ; breadth of outer whorl 

 on the upper side, 08 inch; breadth of same on the under side, 0.14 inch. 



Although at one time inclined to believe that this shell might be a Poly- 

 gyra with the lip broken away, a more careful examination of specimens 

 showing more clearly the umbilicus and some of its other characters, leads 

 me to think that we were nearer correct in first referring it to Planorbis ; or, 

 in other words. 1 hat it more probably belongs to the group Bathyomphalus. 

 Indeed, it seems to be very nearly allied to the existing type of that section, 

 B. contortus (— Helix contortus, Linn.). 



Locality and position. — From the Judith River group, near the mouth 

 of that stream, on the Missouri, in Montana; probably belonging to the 

 closing part of the Cretaceous period. 



