and, indeed, all over the Central Province. The finding of Papilla Blandi, heretofore known only as a fossil in Missouri River drift, living and abundant, is an instance worthy of special mention.
It would seem, then, that a range of high mountains, or any number of ranges would not offer a serious obstacle to the migration of land mollusks, or an insurmountable one to fresh-water forms. The widespread dissemination of such slow-moving creatures is a curious argument for the length of time that the country must have remained in substantially its present condition.
The Sierras of which I have spoken are those which encircle Baker's Park and the San Juan mining region, and extend westward to the base of the great Uncompahgre Mountains, which trend northward not far from the Utah line. This group of volcanic and quartzite peaks constitutes the highest land anywhere in that region, and gives source both to the Rio Grande del Norte and to the head-waters of the Great Colorado River. Its steep southern sides are gashed with tremendous gulches through which the Rio las Animas, the Rio La Plata, the Rio los Mancos, and other streams, which go to make up the Rio San Juan, flow out into the terrible cañon-cut deserts that stretch away across Arizona to the Gila River. For a few miles after emerging from their rocky gates, these rivers water beautiful and fertile valleys, which are cut through the sandstones upturned against the intruded peaks, and which abound in springs. In these valleys are plenty of timber and undergrowth, the climate is rarely cold enough for snow even in winter, and there I expected to gather a rich conchological harvest. In this I was not disappointed, only regretting that I could not make a more thorough examination than was permitted by the rapidity of our travel. Between the Animas and La Plata the trail passes through a valley between the lowest of the foot-hills, where there is a pond of several acres extent, resorted to by all sorts of wildfowl, inhabited by many forms of amphibious life, and choked with an exuberant aquatic vegetation. Here were found thousands of limneas of several species, and quantities of the common Planorbis trivolvis showing a large range of variation among themselves. Like the limneas, the planorbs were extremely fragile in texture, which may be owing partly to the soft bottom, and partly to the scarcity of lime in the water; and they were distinguished by a short vertical diameter, which peculiarity, also, may have been acquired by them from the necessities of their habitat, since snails having shells with small breadth of beam could most advantageously pass between the stalks of standing water-plants that everywhere crowded the pond. But the astonishing fact about this pond was, that on the shore were found perfect specimens—although dead—of the marine genus Truncatella, a broken specimen of an Arca, and living crabs pronounced by Prof. Sidney I. Smith, of New Haven, to be true salt-water forms belonging to the family Astacidæ, That these are survivors of the period, prob-
