476 UNITED STATES GEOLOGICAL SURVEY OF Till'] TERRITORIES. 



A critical comparison of our specimens with Professor Tuomey's very 

 brief description of his A. lobatus might lead to doubts whether it can be 

 the same, since he describes the lateral lobes as "terminating in large bilobed 

 cells." I think, however, that it is almost certain that he mistook the lateral 

 sinuses- for lobes; a mistake that might have been inadvertently made, his 

 specimen being a mere fragment; while if we read it ."lateral sinuses,'' 

 instead of lobes, the description would agree exactly with the medium-sized 

 specimens of the form under consideration. In addition to this, no species 

 of the old genus Ammonites, so far as my knowledge extends, ever has lobes 

 of the form described by him. His description of the siphonal (dorsal) lobe 

 as being "finely serrated" is also inapplicable to this shell. Still, I must 

 think this due to some oversight, or to the imperfection of his specimen, 

 because, of all the collections that I have yet seen from the Southern States, 

 no form agrees so well with his description as this, if we read ' lateral 

 sinuses" for "lateral lobes." I likewise have the impression that Professor 

 Tuomey identified one of our Upper Missouri specimens with his species, on 

 seeing it at Albany many years since. 



A careless observer might, on a hasty examination, mistake this shell 

 for P. placenta, Dekay (sp.) ; but its sharp periphery, smaller umbilicus, 

 and the differences in the nature of its septa will at once distinguish the two 

 forms on comparison. It is true that in large specimens the periphery 

 becomes less acutely angular; but it is never narrowly flattened, as in young 

 and medium-sized specimens of Dekay's species, nor nearly so obtuse as 

 we see in very large examples of that shell, with the flattening nearly 

 obsolete. 



Locality and position. — Professor Tuomey's type of his species A lobatus 

 was from the Cretaceous rocks of Noxubee County, Mississippi. I have also 

 seen a large, fine specimen of it ten inches in diameter, from Pontotoc, Missis- 

 sippi, in the Smithsonian Museum at Washington City. Dr. Owen figured a 

 very small, young example of it, under the name Ammonites hnticularis, 

 from the Fox Hills. Dakota. The small specimen figured on our plate 24, as 

 well as another larger one in the Smithsonian collections, measuring ten 

 inches in its greatest diameter, came from Moreau River, not far from the 

 same locality, where it occurs in the Fox Hills group of the Upper Missouri 

 Cretaceous series. It also occurs in the Cretaceous beds of New Jersey, 

 and at the foot of the Rocky Mountains west of Greeley, Colorado. 



