STANTON. 1 



STEPH ANOCERATID^E. 181 



wide, narrower, and truncated at its posterior end, with a very few 

 shallow sinuosities along its lateral margins; third lateral sinus hardly 

 half as long or wide as the second, and merely faintly bilobate at the 

 end; third lateral lobe a little oblique, simple, and smaller than one of 

 the principal terminal dictations of the first lateral lobe. 



"Greatest diameter, 1.10 inches; convexity, about 0.2G inch. 



"The little specimen from -which the foregoing description was made 

 out is doubtless a young shell. If not, it would uot properly go iuto 

 the group Mortoniceras, as its costse and periphery are without nodes; 

 the former being also more curved forward at their outer ends than is 

 usual in the typical species of the same. Its eostae, however, on the 

 outer volution show rather a distinct thickening at both extremities, 

 and it is very probable that in larger adult individuals distinct nodes 

 are developed. The lobes and sinuses of the septa would doubtless at 

 that size be found more deeply divided and branched. 



" It is on the. supposition that this shell is a young example, in which 

 the usual characters of the group Mortoniceras have not been fully de- 

 veloped, that I here refer it to that group. It might even be a young 

 of the last, or of M. vespertinum. 



"It is evidently not a young specimen ot Prionotropis woolgari, as its 

 cosfce are broader and decidedly straighter than those of that species 

 of the same size, and show no traces of the double node usually seen 

 at their outer ends, even in smaller examples of the same; while its 

 keel is also without the obscure crenulations of that shell, and the 

 volutions less compressed. It is true that the crenulations of the keel 

 in specimens of P. woolgari of this size are not well developed; but 

 traces of them can usually be seen near the larger extremity of the 

 outer turn at that stage of growth. 



" Locality and position. — Mouth of Vermilion river, Nebraska, on the 

 Missouri; where it was found in tire Fort Benton group of the upper 

 Missouri Cretaceous series." 



STEPHANOCERATID^E. 

 Genus ACANTHOCERAS Neumayr. 



ACANTHOCERAS % KANAUENSE 11. Sp. 



PL xxxvi, Figs. 6-8. 



Shell discoidal with convex, slightly embracing whorls whose breadth 

 is greater than the height; umbilicus about equal to the shorter diame- 

 ter of the outer whorl; abdomen broad and gently convex with three 

 rows of closely arranged rounded nodes, one on the median line and the 

 others on either side half way between it and the peripheral margin. 

 On the outer whorls the nodes of the median row tend to unite into a 

 low keel. Sides of the whorl somewhat flattened; costae on the earlier 

 whorls numerous, angular, unequal in size and strongly curved for- 

 ward in passing from the umbilicus to their termination at the outer 

 rows of the abdominal nodes. On about the second whorl a few nodes 



