12-2 PALAEONTOLOGY OP THE UPPER MISSOURI. 



regular curve of the inner turns, and the aperture again tm*ned back towards the 

 body of the shell. In the position of the siphon, the structure of the septa, and in 

 ornamentation, there is no difference between these groups ; and it was probably 

 only at maturity that the shell of a Scaphite dift'cred from that of an Ammonite, 

 while in some species this dilFerence is very slightly marked. 



The genus Ammonites, as here defined, was introduced at near the close of the 

 Triassic epoch, though several authors improperly refer to it some of older Goniatites.' 

 It is very numerously represented through the Jurassic and Cretaceous series, some 

 eight hundred or more species having been already described from these rocks. As 

 might be expected, the species of so large a group present great diversities of form 

 and ornamentation, and various attempts have been made to group them into sec- 

 tions or subgenera, without any great degree of success. When we observe the 

 remarkable difl'crences, however, presented by the form oi the aperture, and the 

 labial appendages of some of the species when found entire, we are led to suspect 

 that we may some time be able to separate them into several natural groups, either 

 having the rank of genera or subgenera. We have no authentic evidence of the 

 existence of this genus after the close of the Cretaceous epoch. 



Ainmonites cordifoi'inis. 



(Plate V, Fig. 2, a, b, c, d, c.) 

 Ammonites cordi/ormis, Meek & Hatoen, Proceed. Acad. Nat. Sci. Phila. March, 1858, 57 ; ib. Oct. 1860, 418. 

 Comp. A. cordalus, Sowerby, Min. Con. toI. I, 1812, 51, 17, figs. 2-4; also D'Okbignv, Pal. France I, pi. 193 ; Geol. 

 Russia, pi. 34, figs. 1-5. 



Shell lenticular, adult specimens being much more convex than the young; umbilicus rather small, or from 

 one-third to one-halt the breadth of the outer whorl ; dorsum carinate ; volutions increasing so as to more than 

 double their diameter every turn, each of the inner ones from one-half to three-fourths hidden within the ventral 

 groove of the succeeding whorl. Surface ornamented by numerous small flexuous costs, which, in crossing the 

 Bides, increase by division and intercalation so as to number two or three times as many at the periphery as 

 around the umbilicus. In approaching the dorsum, they curve forward, and all cross the dorsal carina, to which, 

 in young specimens, they impart a distinctly crenate outline. 



Greatest diameter of a specimen divested of its outer whorls, 3.30 inches ; diameter of its last turn, from 

 umbilicus to dorsum, 1.63 inch ; breadth of same, 1.46 inch. 



The septa are not very closely crowded, and have each five lobes on either side, 

 none of which are deeply divided, or very complex in their details. The dorsal lobe 

 is a little wider than long, and has two principal branches on either side, the two 

 terminal of which are slightly larger than the others, and each provided with seven 

 or eight unequal digitations. The dorsal saddle is about the size of the superior 

 lateral lobe, contracted in the middle, and divided at the extremity into some four 

 or five short, unequal, sinuous, and digitate branches. The superior lateral lobe is 

 as long as the dorsal lobe, but narrower, conical in form, and ornamented with three 

 or four lateral branches on the dorsal side, and two or three smaller ones on the 

 ventral side ; while its terminal branch is bipartite, and its margins, as avcU as those 

 of all the other principal divisions, are more or less sinuous and digitate. The 

 lateral saddle is smaller than the superior lateral lobe, and has on each side three 

 or four very short, obtuse subdivisions, with sinuous margins. The inferior lateral 



' The species in the Upper Trias have more .siraple septa, and often closely approach the genus 

 Ceratites in this respect. 



