250 CALIFORNIA ACADEMY OF SCIENCES. 



decrease with settlement of the country. The chances, how- 

 ever," seem in favor of their being either cave-dwellers or 

 more northern species, introduced where found by birds, 

 but not able to increase there. 



5. LIVING SPECIES. 



While on the subject of introduced species, 1 may remark 

 that the Lim'ax (Amalia) Hewstonl is undoubtedly increasing 

 about gardens, where it is sometimes quite destructive. In 

 1879, Dr. Anderson sent me some from Santa Cruz, where I 

 saw none in 1865. They are also strictly limited to gardens 

 and never found along creeks where several other slugs are 

 common, and widely spread by freshets. It is still unde- 

 cided whether this is the L. Sanclwichensis of the islands. 

 Since I described it and Alexia setifer (= myosotis var.) in 

 1872, the Mya arenaria, first seen in 1874, has become very 

 abundant in San Francisco Bay, from introduction with 

 Eastern oysters, showing how some mollusca may be colo- 

 nized, and it is possible that the Alexia came the same way. 

 It is now abundant on salt marshes near Haywards, where it 

 could not have existed in 1870, when Dr. Yates found all the 

 other marsh shells there. Hemphill also found it in Hum- 

 boldt Bay in 1876. The progress of our local knowledge of 

 the land shells is shown by the additions to the table I pub- 

 lished in 1870 of those found around San Francisco Bay. I 

 then gave 27 species and sub-species, of which 19 were 

 found on west side, and 11 on the east. They now number 

 22 on west side, and 21 on east side, including 7 without 

 shells since added, and reducing the number of sub-species 

 by omitting 6. 



Previous to 1869, it was supposed that none of the large 

 species of land shells existed above the limestone formation, 

 which was then limited to about 4,000 feet elevation on the 

 west slope of the Sierras. Although scarce and stunted at 

 higher points, they have since been found as far up as the 

 tertiary detritus is extensive, which seems to coincide 



