578 



AMMONITES 



off as bifurcations near the middle of the surface of the whorl, while the more prominent ribs originate at 

 the umbilicus ; all cross the dorsum in arches, the convexity of which is directed towards the mouth of 

 the shell, fig. 3, a. The tubercles form four distinct concentric rows, and two obscure rows next the 

 umbilicus; the two middle rows closest together; those near the umbilicus widest apart; row on the 

 periphery most prominent. Diameter two inches. The original nacre of the shell is still preserved, with 

 the lustre, appearance, and iridescent hues of mother-of-pearl. 



Relations and differences, — This Ammonite is somewhat allied in its general aspect to A. Conradi, PI. 

 16, fig. 1, described in Morton, " Synopsis of the Organic Remains of the Cretaceous Group;" but a com- 

 parison of authentic specimens in the collection of the Acad. Nat. Sci. of Philadelphia, shows that A. 

 Nebrascensis is a third larger, the ribs more numerous and closer together, and has one more row of 

 tubercles, the whole shell much flatter, and the ribs on the dorsum much more distinct. Dr. Morton 

 considered figs. 2 and 3, on PI. 1G, of his Synopsis, as also fig. 4, PI. 19, of the same work, as all varieties 

 of the same species, A. Conradi, and remarks, page 39, " So variable, however, are its forms, that I 

 was at first disposed to consider myself in possession of three species, which, however, upon a comparison 

 of upwards of fifty individuals, appear to have their essential characters in common. The most remark- 

 able of these varieties are PI. 16, fig. 2, and PI. 19, fig. 4." 



After having carefully examined the specimen, in the collection of the Academy of Natural Sciences, 

 from which the figures in the above work were drawn, I have no hesitation in saying that figs. 2 and 3, on 

 PI. 16, and fig. 4, on PI. 19, are perfectly distinct specifically from fig. 1, PI. 16; and, if Scaphites* 

 were a satisfactory genus, fig. 3, PI. 16, and 4, PI. 19, at all events, would certainly fall into that genus, 

 since the last half whorl in these is destitute of sutured scptfe, while in fig. 1, PI. 16, they can be 

 traced to the termination. D'Orbigny placed them all, in his " Prodrome," under the genus Scaphitcs, 

 evidently from observing the recurvature of the last chamber in fig. 3, PI. 16, of Morton's Synopsis, and 

 hence that author referred all these figures to the same species (Conradi). Fig. 1, PI. 16, is, however, in 

 every respect a true Ammonite. The others will constitute two new species, for which we may select 

 ijidosus for fig. 2, PI. 16, and navicular is for fig. 4, PI. 19, — specific names which Dr. Morton himself pro- 

 vided, apparently in anticipation of future observations discovering characters and differences sufficient to 

 establish them as distinct species. 



A. Nebrascensis is one of the most common species of the cretaceous formation in the Fox Hills of 

 Nebraska, which form the dividing ridge between the Cheyenne and Moreau Rivers, western confluents 

 of the Missouri River, as indicated on the small map of that region. 



AMMONITES (NEBRASCENSIS ?). 

 (Tab. VIII., fig. 2.) 



Though this Ammonite bears considerable analogy to A. Nebrascensis, fig. 3 of the same plate, yet 

 there are distinctions which may perhaps be sufficient to establish a new species. The shell has a greater 

 diameter — two and one-quarter inches, — the ribs are fewer, wider apart, and bifurcate less frequently ; 

 the umbilicus is larger ; rows of tubercles more obscure ; outer chamber less expanded. Without having 

 a greater number of individuals to compare, it would be unsafe to decide whether this Ammonite should 

 be regarded as a distinct species. This Ammonite occurs with the preceding at the Fox Hills locality. 



AMMONITES CHEYENNENSIS. (N. S.) 

 (Tab. VII., fig. 2.) 



Specific character. — This species is not unlike a young A. Nebrascensis, but it is probably entitled 

 to rank as a distinct species, on account of a peculiarity not observed in this or any other species hitherto 

 described, i. e., the deflection outwards of the inner margin of the whorl that bounds the umbilicus, and 

 in an additional row of tubercles. 



Shell small (one to two-tenths of an inch in diameter), convex, enlarging rapidly; inner margin of 

 whorl deflected outwards. Prominent tubcrculated costa;, which bifurcate either near to the periphery, 

 or two-thirds of the distance from it, passing over the dorsum of the shell in arches curved upwards. The 



* For remarks on genus Scaphites, see under t.lie head of Scaphit.es in this Appendix. 



