The cliff swallow[20] is here very frequent, as well as in all the rocky country near the mountains. This species attaches its nest in great numbers to the rocks in dry situations, under projecting ledges. The nest is composed of mud, and is hemispherical, with the entrance near the top somewhat resembling a chymist's retort, flattened on one side, and with the neck broken off for the entrance. This entrance, which is perfectly rounded sometimes, projects a little and turns downward. It is an active bird, flying about the vicinity of the nest in every direction, {236} like the barn swallow. In many of the nests we found young hatched, and in others only eggs. A fine species of serpent[21] was brought into camp by one of the men. It is new, and seems to be peculiar to this region. A very beautiful species of emberiza[22] was caught; it is rather smaller than the indigo bunting, (emberiza cyanea) with a note entirely dissimilar. It was observed to be much in the grass, rarely alighting on bushes or trees. We also captured a rattle-snake,[23] which, like the tergeminus, we have found to inhabit a barren soil, and to frequent the villages of the arctomys of the prairie; but its range appeared to us confined chiefly to the vicinity of the Rocky Mountains. Its rattle is proportionally much larger than that of the species just mentioned, and the head is destitute of large plates. It seems, by the number of plates and scales, to be allied to the atracaudatus of Bosc and Daud, but their description induces the conclusion that their species is entirely white beneath.[24] It is also allied to the crotalus durissus, L. (C. rhombifer, Beaur.), but it is smaller, and the dorsal spots are more rounded. A specimen is placed in the Philadelphia Museum. The only specimens of organic reliquiæ from this vicinity, which we have been so fortunate as to preserve, are very indistinct in their characters, and are only impressions in the gray sandstone. One of them appears to have been a phytoid millepore, and the other a sub-equilateral bivalve, which may possibly have been a mactra. It is suborbicular, and its surface is marked by concentric grooves or undulations. At a previous encampment numerous fragments of shells, of a dusky colour, occurred in the same variety of sandstone, and amongst these is an entire valve of a small species of ostrea, of a shape very like that of a pinna, and less than half an inch in length. We have a specimen, from another {237} locality, of a very dark coloured compact, and very fœtid impure limestone, containing still more blackish fragments of bivalves, one of which presents the form of a much arcuated mytillus? but as the back of the valve only is offered to examination, it may be a chama, but it seems to be perfectly destitute of sculpture. Another specimen from the mountains near the Platte river, is a reddish brick-coloured petrosiliceous mass, containing casts and impressions of a grooved terebratula.