hannibal: califoenian freshwater uollusca. 113 



Composition of the Fauna. 



The Califoruiau fauna, a summary of which is given in tlie 

 accompanying tables, is a composite one and can only be understood 

 through the Tertiary palaeontological history of the region, which 

 may be summarized as follows : — 



1. Older Eocene: conditions West Indian, great interior lakes, 

 fauna similar to that of contemporaneous deposits in Rocky Mountains, 

 not closely related to any existing American faunas. (Payette, 

 Truckee.) 



2. Younger Eocene, Oligocene : climate continuing nearly tropical 

 and moist, widespread estuary conditions along coast ; fauna similar 

 in aspect to preceding, marked, however, by an invasion of Unioids 

 belonging to recent European genera. (Tejon, John Day.) 



o. Miocene : conditions similar to those of Mexico, arid period, 

 a few minor lacustrine deposits iuterbedded with volcanic ejectamenta, 

 fauna consisting of Gulf States or Mexican genera, a few widespread 

 living species tirst appear. (Mascall, llosaniond, Contra Costa.) 



4. Pliocene : introduction of sub-boreal conditions, slightly colder 

 than present, period of extensive lakes ; peculiar portion of existing 

 Californian fauna appears abrupth' in nearly its present form, wide- 

 spread recent molluscs few, however; half or more of species living; 

 i'f/rf/ulopsis represented by several peculiar species. (Kettleman, 

 Santa Clara, Caclie, Idaho.) 



5. Quaternary, Ilecent : existing conditions, temporary periods of 

 widespread lakes; faunas occupied present or approximately present 

 distribution south of limits of glacial ice-sheet. JN'orth of lat. 50° the 

 fauna contains no peculiar forms excejit in portions of Alaska which 

 were unglaciated, merely more hardy and readily distributed species 

 which have immigrated from unglaciated areas since the middle 

 Quaternary. Most of the species which range beyond the limits of 

 the Californian Pjovince first appear at this time. (Lahontan, 

 Bonneville, Le Conte, Yukon loess, various fluvial and spring deposits, 

 species washed into marine terraces along coast.) 



The living fauna is made up of four elements : (1) Peculiar species, 

 most of which have come down from the Pliocene {PompJwhj.r ejf'um, 

 Amhloxus plicifenis). (2) Species common to central Euro])e and the 

 less Arctic portions of Californian Province which ajjpear to have lived 

 on from a late Miocene migration across Bering Straits [Mar gar if ana 

 mar gar it if era (Palaearctic origin), Anodonta cygnea (Californian origin). 

 (3) Species common to Alaska and adjacent portions of Sibeiia, 

 apparently remnants of a Quaternary migration across Bering Straits 

 [Anodonta cygnea Beringeriana, probably Californian origin). (4) Species 

 of European or American origin which have immigrated from the east- 

 ward at various periods, chiefly during pre-Bonneville-Laliontan times. 

 The European species in the American Province probably date back 

 to a middle Miocene migration {Physa hypnorum (European origin ?), 

 I'lanorhis antrosns (American origin)). On the other hand, several 

 species of Californian origin have extended their ranges southward or 

 eastward beyond the limits of the Californian Province {Margaritana 

 mnrgaritifera fulcata, Lymnaa solida Cuhensis). 



