TO JUNCTION OF GRAND AND GREEN RIVERS. 133 



Greatest diameter of a specimen retaining only a small portion of the non-septate 

 outer whorl, 4.40 inches; greatest convexity of same, 0.95 inch; breadth of umbilicus, 

 l..'>5 inches; breadth of last whorl from the siphonal to the umbilical side, 1.80 inches. 



Young specimens of this species differ materially from the adult ; those not more 

 than an inch or an inch and a half in diameter being destitute of nodes, and provided 

 with a continuous instead of a waved carina. Their costse are also more crowded, 

 more numerous, and curve distinctly forward near the peripheral margins. At this 

 si/<>, the shell closely resembles specimens of Ammonites percarinatus, Hall and Meek, 

 of the same age ; but, as it advances in growth, it assumes quite different external 

 characters, while its septa differ at all ages. 



It seems to be related to a Texas species, described by Dr. Shumard, in the Trans- 

 actions of the St. Louis Academy of Sciences (vol. 1, p. 539), under the name of A. 

 Gmysoncnsis. Dr. Shnmard, however, to whom I sent sketches of it, regarded it as 

 clearly distinct. He wrote that he had seen quite a number of specimens of A. 

 (lni//H'>iir'.iif<is, and that none of them attain more than one-third the size of the form 

 under consideration, while they are always destitute of nodes, and have a more dis- 

 tinctly-waved c;irnia. 



I am far from being satisfied that this shell belongs to the same group as that for 

 which I have, in one of Dr. Hayden's Reports and again in the Upper Missouri Palae- 

 ontology, used the name Prionocydm. It is certainly not a true Ammonites, however, 

 as that genus is restricted to typical forms, but more nearly allied to Prionocydus. 



Named in honor of Capt, J. N. Macomb, of the United States Topographical Engi- 

 neers, commander of the expedition. 



Locality and position. Banks of Canadian; lower part of Middle Cretaceous of 

 New Mexican section. 



