ably comparatively recent, when here was a soft-water marsh that remained caught in this basin among the hills after the country, for a long distance south of it, had become dry land, seems very evident. It is difficult otherwise to account for their presence.
Farther on, in the valley of the Rio La Plata, where it emerges from its magnificent quartz cañon, and where the gold placer-mines and prospective city of La Plata are situated, a fine collecting-ground was found. This was so far south that many deciduous trees grew in the river-bottoms, and nearly every terrestrial species hitherto met with was there to be had in plenty. For the next ten days we were entirely in the lava-blasted, treeless and waterless deserts on the northern margin of the Rio San Juan, engaged in exploring the vestiges of that ancient semi-civilized race of village Indians, the remnants of which still exist in the small tribe of Moquis on the Little Colorado. During this time no mollusks were found except, where there was a little moisture, a few pupas, which seem able to live anywhere, and many bleached shells of various species that had been drifted down from the mountains at times of high water.
Our return-journey from the San Juan country was made from its very sources along the course of the Rio Grande. It led us through Antelope Park, on the eastern side of which lies St. Mary's Lake, a beautiful little sheet of crystal water studded with islands, and held among precipitous cliffs that afford it no visible outlet. It seems to be merely a great rocky basin, holding the melted snows of the surrounding heights. Its surface is over 9,000 feet above the sea. There existed in countless numbers in this lake a large species of coil-shell which was a nondescript, and which I have since named Helisoma plexata. Each of the hundreds of individuals seen possessed in a more or less marked degree a twisted appearance, resulting from a change in the plane of revolution in old age, which is the most striking specific character. This sudden change in the directness of the growth causes the carina of the third whorl to rise into a sharp shoulder on the right side, while on the opposite side the third whorl sinks underneath the overflowing outer whorl. A similar change often occurs in the fourth whorl, giving a braided look to the shell. How this species came almost alone to inhabit this secluded lake is a problem, complicated by the fact that probably there is not another large Planorbis within fifty miles. That the wild-fowls, abundant on the lake, brought the eggs clinging to their feet, may be a plausible explanation; but where did they bring them from, and when? The bottom of the lake is, for the most part, rough conglomerate rock, and is in many places filled with tangled water-plants, which may partially account for the peculiarities of the species. The shells of this genus appear to be especially subject to distortion under abnormal conditions.
Continuing our course down the valley of the Rio Grande to the town of Del Norte, we there left the river and struck across the San
