The Aquatic Molluscs of Southern California and 

 Adjacent Regions, A Transition Fauna. 



Constituting Part II of "A Census of the Land and Fresh- 

 water Mollusks of South-western California.''* 



Harold Hannibal. 



GENERAL REMARKS. 



The term " Calif ornian Province'" was introduced by Wood- 

 ward + to embrace the portion of North America lying north of 

 Mexico and west of the Rocky Mountains, distinguished by a 

 peculiar fauna of land and fresh-water mollusks. It was sub- 

 sequently recognized by Tryon. Fischer, and Cooke in their 

 more recent manuals and is now generally accepted; constitut- 

 ing the western portion of the Xearctic Region. The first 

 writer to attempt a subdivision of this extensive area into con- 

 venient sub-units was Dalli who recognized in the region from 

 the Columbia Basin northward three •'systems" or faunules. 

 In 1910, the writer after a study of nearly all the available 

 records extended this classification to the less boreal portions 

 of the Province lying within the United States. Additional 

 study has shown the necessity of modifying these divisions 

 somewhat, due to unreliable records', but in the main the con- 

 clusions reached in "West Coast Shells" have proven well 

 founded. 



From these Systems the writer has selected two. the Los 

 Angeles, embracing with one or two exceptions to be noted 

 later, the Pacific drainage from Point Conception, California, 

 to the vicinity of San Sebastian Viscaino Bay, Baja California, 

 and the Arizona, including the catchment area of the Colorado 

 River below The Needles or thereabouts, the drainless basin 

 of the Salton Sea, and the desert region about the head of the 

 Gulf of California, as a subject for special study, no less on 

 account of the local interest which this region holds to the 

 numerous southern Californian conchologists than that it eon- 



*The removal of the writer from Stanford University to the Wash- 

 ington State Museum before the completion of the manuscripts of the 

 summary and additions to the landshell fauna which were to have found 

 a place in the following pages has necessitated their omission at this 

 time. The scope of the present part has been enlarged somewhat to 

 embrace the fauna of the lower Colorado Basin, and the article is com- 

 plete in itself. A discussion of relationships, additional records, and 

 the accompanying bibliography of the landshell fauna may appear in 

 the future as Part ill. 



+ Manual of the Mollusea, 1856. III. map facing title page. 



iPop. Sci. Mo. LXVI. 1905. 362: Alaska XIII. 1905, 2. 



18 



