Luis Valley to Mosca Pass through the Sangre de Cristo Range. This alkali and sage-brush plain, fifty miles wide, is very far from being "the garden of the world," as it has been styled. Near the eastern side is a group of lakes, the water of which is highly alkaline. These lakes are the abode and breeding-place of wild geese and ducks in the greatest number, which are tormented without end by the gulls that also make the lakes their home. On the gravelly beaches I picked up many shells, and doubtless in the deep water many more species might have been dredged, had there been time. But nowhere were there any bivalves, except the little cyclades. The fact that there was no lack of molluscan life in these intensely bitter waters was not surprising, since mollusks seem to flourish in mineral springs of both hot and cold water everywhere. We had seen before a fine illustration of this adaptation to peculiar conditions. The Grand River, which flows through Middle Park, contains no mollusks at all that I could discover; but at Hot Springs, in a little lagoon filled at high water, large, clear, ampullacea-like forms of the familiar Physa heterostropha were common. Close by, in the few yards of exposed outlet of the springs of hot sulphur-water from which the locality derives its name, there occurred in the greatest profusion a blackish, globose variety of the same species only one-fifth of an inch long. The temperature of this water was at some points as high as 100° Fahr. In the basin of a still hotter spring not ten feet away, whose waters were saturated with chlorides of sodium and magnesium, hundreds of still smaller Physæ were-floating about in mats glued together by a tangle of confervoid vegetation and the depositions of the water. All these seemed to have lost their apices by erosion, "which is extremely liable to happen to shells living in water charged with alkaline salts other than lime." On the other hand, quite as small and black were the examples from the pure cold springs near Saguache, where there was seemingly nothing whatever to stunt their growth.
I was stimulated, by the results of my study of my own collection from Colorado, to gather all possible information about the mollusks of the Central Province generally, as it has been limited above. The bibliography was quite large, but the notes of locality and station very meagre. Tabulating the sum of the information open to me, and including my own summer's work, I found that 138 nominal species had been recorded as occurring in this inter-montanic region. Of these, 49 were also Californian species; 15 occurred also in the Eastern United States; 8 hailed from the Colorado Desert; 7 were found all over the continent, and 8 all over the world; and 3 belonged in the Eastern Province, west of the Alleghanies only. This left 47 nominal species, whose range, so far as yet known, is confined to the Central Province. Many of the specific names in this list, however, rest upon very insecure foundations, and will, no doubt, soon be reduced to synonyms. With respect to their vertical distribution, ob-
