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 twoinches. 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, Pl. 
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 Pl. 16, of his Synopsis, as also fig. 4, Pl. 19, of the same work, as all varieties 
of the same species, 4. 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 Pl. 16, fig. 2, and Pl. 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 
Pl. 16, and fig. 4, on Pl. 19, are perfectly distinct specifically from fig. 1, Pl. 16; and, if Scaphites* 
were a satisfactory genus, fig. 3, Pl. 16, and 4, Pl. 19, at all events, would certainly fall into that genus, 
since the last half whorl in these is destitute of sutured septe, while in fig. 1, Pl. 16, they can be 
traced to the termination. D’Orbigny placed them all, in his “ Prodrome,” under the genus Scaphites, 
evidently from observing the recurvature of the last chamber in fig. 3, Pl. 16, of Morton’s Synopsis, and 
hence that author referred all these figures to the same species (Conradi). Fig. 1, Pl. 16, is, however, in 
every respect a true Ammonite. The others will constitute two new species, for which we may select 
gulosus for fig. 2, Pl. 16, and navicularis for fig. 4, Pl. 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. 
. 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. VIIL, 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. 8.) 
(Tab. VIL, fig. 2.) 
Specific character.—This species is not unlike a young A. Nebrascensis, but it is probably entitled 
to rank asa 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 tuberculated 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 the head of Scaphitfes in this Appendix. 
