OF NEBRASKA. 



579 



tubercles of the coshis 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. S.) 

 (Tab. VIII., 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. 

 G, a). Umbilicus perforate ? Serrations of the sutures represented by fig. 6, a. 



G reatest 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 aiford magnificent cabinet specimens. This species is associated, 

 at the Great Bend of the Missouri, with Inoceramus Cripsii, in the cretaceous formation of Nebraska. 



AMMONITES MOREAUENSIS. (N. N.) 

 (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 Ammon ite 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. VII., and some almost micro- 

 scopic univalves. 



AMMONITES LENTICULARIS. 

 (Tab. VIII., 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. VII., fig. 10. 



Relations and differences. — This species differs from A. placenta of Dckay, not only in size, but also 

 in the serrations of the septa), and in the umbilicus being much smaller and imperforate. 



SCAPIIITES 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 Scapjhitcs and Ammonites. 



In 1811, Parkinson established the genus Scaphitcs, and gave the following generic description of it. 



" A fossil concamcrated 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 



