NATURAL SCIENCES OF PHILADELPHIA. 435 



agencies, into numerous deep valleys and ravines, so as to leave various 

 peaks, isolated columns, towers, &c, presenting, as seen from a distance, ex- 

 actly the appearance of the ruins of an ancient city. The difficulty the travel- 

 ler meets with in finding his way through this interminable labyrinth, had 

 caused the Indians to call it in their own language, the Bad Grounds, hence 

 the French name Manvaises Terres, applied by the Canadian Voyageursin the 

 employ of the Fur Companies. 



The vertebrate remains found in these beds belong to the genera Oreodon, 

 Agriochrerus, Pazbr other ium, Leptomeryx, Leptauchenia, Protomeryx, Merycodus, 

 Titanotherium, Leptochaerus, Hyracodon, Entelodon, Paheochoerus, Rhinoceros, 

 Steneojiber, Machairodus, Anchitherium, Hyopotamus, Hyanodon, Ischyromys, 

 Palaolagus, and Eumys, Testudo, &c, &c. The affinities of these fossils, as lias 

 been shown by Prof. Leidy, clearly establish the Miocene age of this formation. 



Comparatively few invertebrate remains have yet been found in the White 

 River Group. They consist of one species of Helix, one or two of Limncea, a 

 small Physa, two or three small species of Planorbis, &c. No fossil leaves, nor 

 beds of Lignite, have been met within it ; and all the animal remains, as may 

 be seen from the foregoing list, are terrestrial and fresh water types. 



The Loup River Beds consist mainly of incoherent materials, and were evi- 

 dently deposited after the upper surface of the White River Group had been 

 worn into ravines and other depressions. It occupies much of the surface of 

 the country in the region of the Loup Fork of Platte River, and extending far 

 south of the latter stream. The vertebrate remains described by Prof. Leidy 

 from it, belong to the genera M.'galomeryx, Procamelus, Cervus, Rhinoceros, 

 Mastodon, Elephas, Hipparion, Merychippus, I'Jquus, Castor, Felis, Canis, Testudo, 

 &c, many of which are very closely allied to recent species. A few shells of 

 the genera Helix, Physa, &c, apparently identical with living species, have 

 also been found in these beds. All the species of vertebrate and other remains 

 yet found in them, are distinct from those occurring in the White River Group 

 and beds below, and they have not yet afforded any brackish or marine types 

 of any kind. 



When we take into consideration the position of this formation above the 

 well marked Miocene White River Group, and the relations of its organic re- 

 mains to Pliocene and recent species, there is little room for doubting the cor- 

 rectness of its reference to the horizon of the Pliocene of Europe. 



SILURIAN (PRIMORDIAL) FOSSILS. 



BRACHIOPODA. 



Genus OBOLELLA, Billings. 



Obolella nana. 



Shell very small, subcircular, or transversely suboval, moderately convex, 

 rather thick ; front broadly rounded ; sides more narrowly rounded. Beak ot 

 dorsal valve short and obtuse. Ventral valve proportionally a little longer 

 than the other, about as long as wide, and having a slightly more prominent 

 beak ; without a distinct mesial ridge within ; scars of aductor muscles ? located 

 behind the middle and diverging towards the front. Surface marked by a few 

 concentric furrows ; exfoliated specimens showing small, obscure, regularly 

 disposed radiating strias on the inner laminae. 



Length of dorsal valve, 015 inch; breadth of do., 0-17 inch; convexity, 

 0"15 inch. Length and breadth of ventral valve of a smaller specimen, each 

 0-14 inch. 



In first sending on to the Academy a description of this little shell, we had 

 referred it with doubt to the genus Obolus, stating, at the same time, that its 

 muscular scars, so far as they could be made out from the only specimen we 



1861.] 



