MOLLUSCA OF THE WEST COAST OF NORTH AMERICA. 347 



conclusion that only one shell is common to east and west, and not even that 

 to the intermediate islands*. 



71. And if we are thus struck with the isolation of the W. American fauna 

 in general, so are we with the separation of its component parts. Let us 

 compare (as being the most unmixed sources of information) the central 

 collection of Prof. Adams at Panama, on the one side with the equatorial 

 collections of Messrs. Cuming and Fontaine, and with the Chilian researches of 

 the former and D'Orbigny ; and on the other with the Gulf collection of 

 M. Reigen, and those in California by Mr. Nuttall and the U.S. Exploring 

 Expedition. We find that, while so large a number of species are common to 

 Mazatlan, Panama, Guayaquil and the Galapagos, that they may fairly be 

 reckoned as one great province, scarcely any are common to the equatorial 

 districts and Chili, and still fewer to the Gulf and San Francisco ; insomuch 

 that on a comparison of known forms between Mr. Nuttall's collection, 

 M. Reigen's, and the W. Indian fauna, it may be safely asserted that there is 

 more in common between the two latter than the two former. 



We proceed now to the details and the exceptions ; merely premising that 

 the student must bear in mind the very unsatisfactory nature of most of our 

 materials, and must therefore receive what follows simply as the approxi- 

 mation partially attainable in the present state of the science, and not as 

 absolute truth. 



72. In the Boreal Fauna, we naturally look for different conditions from 

 those which prevail in the continent generally. The near connexion of Asia 

 and America at Behring's Straits and the Aleutian Islands leads us to ex- 

 pect similar forms on the two continents ; and as the boreal species are known 

 to be both widely distributed and extremely variable, we shall not be sur- 

 prised to meet again with a few familiar European types. 



The following Polar species are quoted from the extreme north at 

 Icy Cape: — 



Corbula gibbosa. 

 Tellina alternidentata. 



inconspicua. 



nasuta. 



Astarte crassidens \ J =corrugata. 



Natica pallida. 

 Buccinum angulosum. 



polare. 



temie. 



Chrysodomus fornicatus. 

 Trophon lamellosus. 



lactea J \ =semisulcata. 



Trichotropis borealis. 



Of these none as yet appear in the Sitcha lists but Tellina nasuta, and the 

 European Trich. borealis. The latter probably reaches Oregon, while the 

 former travels as far south as San Diego. 



73. From the Sitcha district are quoted 102 species (£5 bivalves, and 

 77 univalves); of which 16 are northern forms, not known south of Behring 

 Sea ; 18 biv.-f 26 un. — 44 are found in Asia, principally in the Ochotsk Sea ; 

 7biv.4-12un.= 19are common to Oregon; about the same number, but not the 

 same shells, are found in Upper California, and a few have a wide range. Triton 

 scaber is the only Sitcha Proboscidean which reaches California. The Kamts- 

 chatkian Cryptochiton Sielleri and Placunanomia macroschisma reappear in 

 Upper California, but have not yet been found in intermediate stations. Mytilus 

 edulis reaches from Kamtschatka to Upper, and Tellina nasuta with Cardia 

 Nuttalli and Californiense to Lower California; while Acmcea patina travels 



* Vide Woodward's " Manual of Mollusca," pp. 373 et seq., London, Weale, 1851-56 : a 

 work which combines in a small compass, and at a price within the reach of all, a larger 

 amount both of accurate detail and philosophical research than is anywhere else accessible. 

 The chapters on geographical and geological distribution are invaluable. 



