144 PROCEEDINGS OF THE ACADEMY OP 



species. The variety tenuis, however, is very similar to that of . 

 Binney and Tryon. The figures show the varieties of form and 

 color met with here, but scarcely two specimens are exactly alike, 

 and some adult shells are as ventricose as the young one figured. 



I found this species exceedingly numerous on the stony shore 

 of a tide-water creek within the limits of San Francisco, but onlv 

 for a distance of about ten rods, where a rocky point comes to the 

 shore, though a few can be found for half a mile on loose stones 

 and timbers, crawling under the stones in great numbers when 

 the tide is out long. They look so similar to the buds of the 

 Salicornia which grows about the spot that they may easily be 

 overlooked by enemies when crawling among it. They creep 

 rather rapidly by a steady gliding motion, like the other pulmo- 

 nates, and live for a week or more in a damp vial, though killed 

 quickly by immersion in fresh watei. 



Though so numerous that several hundred may be scraped oil' 

 from the bottom of a small stone, it is strange that this shell has 

 escaped the notice of a host of collectors for more than twenty 

 years, though many of them must have passed within a few yards 

 of the spot, as it is close to the old southern entrance to the city 

 by land. This suggests that it may have been introduced from 

 China on the bottoms of fishing-boats, or as ova in damp nets or 

 otherwise, especially as the Chinese have always made the creek 

 a fishing station, and cultivate the low land along its shores. It 

 is, however, just as likely to be native, and to have been over- 

 looked on account of the limited and rather inaccessible locality 

 it inhabits. Until it is proved that these delicate aquatic mol- 

 lusca can be transported from one country to another by the ways 

 of commerce, it is better to consider them indigenous. I may be 

 able to find them in other similar localities out of the way of 

 foreign vessels. 



It is almost as probable that the Assiminea, which is equally 

 numerous in the same spot, was introduced in the same way, though 

 that is probably the same as found in other similar creeks about 

 the bay. 1 



1 Note on "A myosotis' n of IT. S. Atlantic Coast. The variations of the 

 Pacific shell suggest that the form figured by Binney is only the northern 

 attenuated form of what may occur farther south more fully developed as 

 " Leuconia n Sayii (Kuster sp.), which, according to Jay's catalogue, is 

 found in New Jersey. Conrad's and De Kay's descriptions of "borealis" 



