OF THE UPPER MISSOURI. 
67 
Fishes. 
Xystracanthus arcuatus, Leidy. Upper Carboniferous rocks at Leavenworth landing. 
Cladodus occidentalis, Leidy. Division No. 37 of foregoing general section. At Manhattan. 
Petalodus Alleylianiensis, Leidy. Jour. Acad. Nat. Sci. vol. iii, p. 161. Division No. 10 of foregoing general 
section. Fort Riley. 
CHAPTER X. 
V. Jurassic System. 
The Black hills have up to this time afforded the most satisfactory evidence of the ex¬ 
istence of this system in the West. It is there brought to the surface by the upheaval of 
the older rocks in the form of a belt or zone, five to fifteen miles in width, engirdling the 
principal axis of elevation. The group of rocks, which we have supposed to belong to 
this period, are characterized by beds of variegated argillaceous and calcareous grits, sand 
and sandstone, with seams and beds of gypsum, varying from one inch to twenty feet in 
thickness. The gypsum seems to form a portion of a series of brick-red beds, composed 
of argillaceous and calcareous grits, which give a remarkably picturesque appearance to 
the external features of the country, where these Jurassic rocks are exposed. None of the 
organic remains already discovered, which are quite numerous in species, are positively 
known to be identical with those found in rocks of the same age in the Old World, but 
they belong to the same genera, and many of the species are so closely allied to forms 
characteristic of the Jura of Europe, that we cannot now hesitate to admit this system into 
our series. In a paper by F. B. Meek and the writer, published in the Proceedings of 
the Academy of Natural Sciences of Philadelphia, March, 1858, the following list of fossils 
was given with comparisons showing their affinities to well-known Jurassic forms. This 
evidence I will here repeat with such additional proof as I have been able to secure by 
our investigation of the undescribed fossils in the collection up to the present time. 
1. Pentacrinus asteriscus, Meek and Hayden, is so nearly like the Liassic P. scalaris (Goldfuss), that it is with 
some hesitation we have regarded it as new. 
2. Avicula ( Monotis ) tenuicostata, M. and IL, is very closely related to M. sulstriafa of Munster, from the Lias. 
3. Area ( Cucullcea ) inornata, M. and II., is very similar to C. Munsteri (Zeiten), also from the Lias. 
4. Panopcea ( Myacites ) subelliptica, M. and IL, is similar to the Liassic forms M. liasensis and M. Alduincnsis 
of Quensted. 
