462 UNITED STATES GEOLOGICAL SURVEY OP THE TERRITORIES. 



to typical forms. Should complete undistorted specimens show, as I think 

 very probably will be the case, that it differs generically or subgenerically 

 from Phylloceras proper, I would propose for the group into which it would 

 in that case naturally fall, the name Rhceboceras. 



Locality and position. — One hundred and fifty miles above the mouth of 

 Milk River, on the Missouri, in Montana Territory ; from the Fort Pierre 

 group of the Upper Missouri Cretaceous series. 



Genus PLACENTICERAS, Meek. 



Synon. — Ammonites (sp.), Dekay, Morton, and others (not Brug., as restricted). 



Piacentieeras, Meek (1870), Proceed, Philos. Soc. Philad., XI, 429 ; and (1872) in Haydeu's Second 

 Ann. Rep. U. S. Geo]. Survey of the Territories, 297 (proposed as a section of Ammo- 

 nites). 

 SphenodiscuSjMee'k (1872), ih. (proposed as a section of Ammonites). 



Etym. — Placenta, a cake ; Ccras, a horn. 

 Type. — Ammonites placenta, Dekay. 



Shell usually of large size at maturity, lenticular or discoid in form, with 

 sides converging more or less gradually to the periphery, which is either 

 very narrowly truncated, or cuneate-carinate, and, when truncated, flattened 

 or slightly concave, with its margins angular and smooth, or more frequently 

 each provided with a row of small, compressed, and generally alternating 

 nodes, arranged with their longer diameters in the direction of the peripheral 

 curve ; volutions much broader on a line with the plane of the shell than 

 convex, all deeply embracing; umbilicus small, or very small; aperture 

 sagittate, or nearly so; lip unknown; surface in young examples nearly 

 smooth, or only with sigmoid lines of growth, and in adult shells often one 

 or two rows of small, low, lateral nodes on each side, sometimes also pro- 

 vided with obscure, undefined lateral ridges or undulations ; septa with from 

 about ten to fourteen, comparatively short, generally not very deeply-divided 

 lateral lobes, and as many sinuses, arranged in somewhat undulated rows 

 across each side of the volutions; siplional lobe generally a little shorter 

 than the first lateral lobe on either side; lateral lobes increasing regularly 

 in length to the third one inclusive, and beyond this diminishing in size to 

 the umbilical margin. 



The foregoing diagnosis is intended to include two sections, which, 

 according to the late methods of classification of such shells, might be 

 arranged as distinct genera, if not even more widely separated. These groups 

 may be each defined as follows: 



