CALIFORNIA ACADEMY OF SCIENCES. 243 



ward proving a very recent origin for these species. A 

 deposit of lignite containing numerous extinct forms of 

 fresh-water shells is found at various localities, and shows 

 that conditions suitable for land shells existed at the same 

 time. The southern islands, with their extinct forms of 

 quaternary age, have been before mentioned. I have, how- 

 ever, obtained a few found along the middle of the coast, 

 which differed in smaller size and fewer whorls from the 

 living, indicating a colder climate; also a subfossil Leuco- 

 chila from near San Francisco. 



3. HISTORIC CHANGES. 



It is necessary to give some observations on the histo- 

 rical changes that have occurred in the last three hundred 

 years as affecting the land shells, and to forecast from them 

 the probabilities of increase or decrease in numbers, adding 

 an account of some species supposed to be very recently 

 extinct, and new facts respecting others. 



The long and extreme dry season of California has always, 

 no doubt, been effective in limiting the extension of range 

 of the land shells, though the existence of identical or only 

 slightly varied species over distances of three hundred to 

 six hundred miles north and south, indicates that they must 

 have spread during the existence of a warmer and moister 

 climate in the earlier quaternary. In the Sierra Nevada, 

 the variations found to exist in species are more the effect 

 of altitude than latitude, two species at least being almost 

 identical from Shasta to San Diego. In the Coast ranges 

 proper, however, local variations constituting subspecies 

 are numerous, and many of them separated in range by wide 

 intervals, in which few or none exist. These gaps are evi- 

 dences of either periodical severe droughts, or increased 

 dryness of climate, occurring since that stage of the qua- 

 ternary when the interior valleys had become mostly dry 

 land by filling in from the mountains, and the great bodies 

 of heated water before existing gradually ceased to furnish a 



