504 CALIFORNIA ACADEMY OF SCIENCES. 



sides those last given. Of these I found only No. 11 up to 

 1200 feet elevation. The triangular peninsula, 1436 feet 

 high, west of Tomales Bay, is chiefly of tertiary strata, but 

 in parts granitic, with much sandy and marshy land about 

 Drake's Bay, quite a dense coniferous forest covering much 

 of its surface. Being also exposed to the sea breeze and 

 fogs, it would seem better adapted for the pulmonates than 

 San Francisco County, which has a similar extent, but so far 

 few species have been brought from there. Along the east- 

 erly slope of the ridge a ledge of limestone is exposed, but 

 I could not find any species near it, nor on the higher parts 

 of the peninsula. From the west slope I have received 

 varieties of No. 30, chiefly that often called " Nickliniana" 

 which differs from No. 33 in a thicker shell, with a coarsely 

 wrinkled surface, often blotched with whitish patches. This 

 variation is caused by the influence of the spray from the 

 ocean sprinkling the growing shells among the shrubbery 

 just within the sand hills of the beach, where they are some- 

 times quite numerous, and the same effect is j^roduced on 

 several other species growing in similar situations all along 

 the coast. From this, I infer, arose the reference of ' 'A. 

 NicMiniana'' to San Diego, where a similar variety of A. 

 Kelletii is found. 



Marin County thus has only fifteen forms. This great 

 diminution in number of species where conditions seem 

 favorable for more, will be better understood after describ- 

 ing those of the remaining counties. 



The figures along the sea coast give the heights of almost 

 perpendicular rocky bluffs, which like the steep Farallone 

 Islands, are chiefly of granite or hard metamorphic rock. 

 Angel Island and all the others near the entrance of the 

 bay, are also chiefly of this nature, and have furnished very 

 few Pulmonata, but Mare Island is tertiary, and supplied 

 more. 



Sonoma County. 



Little more than half of this is shown on the map, but 



