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 

 m 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 o.i the other with the Gulf collection of 

 M. Keigen, 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 

 Majzatlan, 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. Keigen 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- 

 TsZte^Jruth^ attainable in the present state of the science, and not as 



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

 those which prev;a,l in the continent generally. The near connexion of Asia 

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



fn tT r •h'?' ^'- th.? *^^°/'^°t'°^°t« •' ^«d 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. 



Icy Cap??!!.'''"^ ^'''''''' 'P'"'"' ^'' ^"^*'^ ^'■""^ '^^ ^^^'^"^^ ^onh at 



Corbula gibbosa. 

 Tellina alternidentata. 



inconspicua. 



■ nasuta. 

 Astarte crassidens \ f =corrugata. 

 — — lactea / \ =semisuleata. 



Tnchoti'opis borealis. 



Natica pallida. 

 Buccinum angulosum. 



polai-e. 



tenue. 



Chrysodomus fornicatus. 

 Trophon lamellosus. 



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

 European Trzch. boreahs The latter probably reaches Oregon, whiie he 

 former travels as far south as San Diego. 



73. Fromtbe Sitcha district are quoted 102 species (55 bivalves and 

 sL""8 b'^ioe 1 " 4f --/-thern forms, not Lownluth of Srtng 

 7biv' il2un -iq«rr "'^ ^A^ '" "^"'t' P""^ip^lly ^^ the Ochotsk Sea° 

 7 biv. + 12 un.l9are common to Oregon ; about the same number, but not the 

 same shells, are found in Upper California, and a fev, have a wide ra^te Trl^n 

 .caber is the on y Sitcha Proboscidean which reaches Califorl. TheKarS 



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

 The chapters on geographical and geological distribution are invaluable! a«"sible. 



