46 THE POPULAR SCIENCE MONTHLY. 



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 laud, 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 canon, 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 ves- 

 tiges of that ancient semi-civilized race of village Indians, the rem- 

 nants 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 any- 

 where, 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 sur- 

 rounding 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 al- 

 most alone to inhabit this secluded lake is a problem, complicated by 

 the fact that probably there is not another large JPlanorbis 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 pecu- 

 liarities 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 



