OF NEBRASKA 579 
tubercles of the costa form five concentric rows; the inner one obscure, and becoming obsolete one-third 
of the distance down the terminal volution. It is associated with the preceding. 
AMMONITES OPALUS. (N. 8.) 
(Tab. VIIL., fig. 6.) 
Specific description.—Shell moderately convex, expanding, but gradually, towards the last chamber. 
Convolutions about two ; last one embracing the preceding in a deep fossa. Surface destitute of tubercles, 
but ornamented with delicate, undulating ribs; every second, third, fourth, fifth, or sixth, more promi- 
nent than the intermediate ones, extends from the umbilicus to the periphery. Intermediate ribs formed 
by bifurcations, which take place about two-thirds and one-half of the distance towards the periphery, 
and, thus multiplied, cross the dorsum. Siphuncle visible where the shell is abraded on the dorsum (fig. 
6,a). Unmbilicus perforate? Serrations of the sutures represented by fig. 6, a. 
Greatest diameter, two inches and a half; greatest thickness, one inch. 
Remarks.—The specimens of this species from the Great Bend of the Missouri have often the nacre of 
the shell in the most exquisite state of preservation, and reflect light from their surface in the richest 
iridescent mother-of-pearl hues. They afford magnificent cabinet specimens. This species is associated, 
at the Great Bend of the Missouri, with Inoceramus Cripsii, in the eretaceous formation of Nebraska. 
AMMONITES MOREAUENSIS. (N. 8.) 
(Tab. VIII. fig. 7.) 
Specific character.—Shell small (one and a quarter inches in diameter) ; convolutions three and a half ‘ 
about two-thirds of each whorl visible in the perforate umbilicus, the rest being concealed in a fossa of 
the terminal whorl ; slightly convex; dorsum slightly flattened. Surface ornamented with undulating 
ribs ; every second or third well defined. Intermediate ribs formed by bifurcations of the main ribs, which 
take place at different distances from the umbilicus. Four rows of small tubercles; the row on the 
periphery most prominent; the row next the umbilicus obscure. 
Relations and differences.—This Ammonite has most analogy to A. Abyssinus of Morton; but it is 
larger; ribs more prominent; convolutions more numerous. Rows of tubercles on the sides of the volu- 
tions as well as on the periphery. 
From the Fox Hills of Nebraska, associated with Avicula, fig. 10, Tab. VIL., and some almost micro- 
scopic univalves. 
AMMONITES LENTICULARIS. 
(Tab. VIIL., fig. 5.) 
Specific character.—Shell small; remarkably thin, compressed dorsum a sharp edge; umbilicus 
very small; surface apparently smooth. Volutions, except the last, entirely concealed in the fossa of the 
ultimate whorl. 
This specimen is not sufficiently well preserved to give a full description of it, but its lenticular form 
and remarkably sharp dorsal periphery serve to distinguish it. 
Dimensions.—One and three-tenths inches. 
From the Fox Hills, associated with the small Avicula, Tab. VIL, fig. 10. 
Relations and differences.—This species differs from A. placenta of Dekay, not only in size, but also 
in the serrations of the septe, and in the umbilicus being much smaller and imperforate. 
SCAPHITES OF PARKINSON. 
Many of the convoluted and camerated shells of Nebraska are, I believe, destined to throw a new light 
on the forms hitherto referred to the genera Scaphites and Ammonites. bo 
In 1811, Parkinson established the genus Scaphites, and gave the following generic description of it. 
“‘ A fossil concamerated shell, commencing with spiral turns; the last of which, after being elongated, 
is reflected towards the spiral part.” He follows this description with these remarks: “The very wide 
