NORTHWEST COAST MARINE SHELLBEARING MOLLUSKS. 7 



culiarity of situs, as in Crepidula, Ilaliotis, Acmaea, and other sessile 

 forms. They are not true species, but further investigation is required 

 to establish the source of their differences. A list of this kind is the 

 basis for improvements, additions, and corrections, and such changes 

 are to be taken as inevitable and welcome. For errors such as are 

 almost impossible to exclude in work depending on such multitudi- 

 nous and scattered data, those who have undertaken similar work will 

 be most charitable. Naturally the utmost care has been taken to 

 avoid them. 



The Pteropods are now known to exist (by the almost miraculous 

 preservation of the characters of the soft parts) in material from 

 the Middle Cambrian collected by Walcott. The absurdity of deriv- 

 ing them from the Opisthobranch forms which first occur in Mesozoic 

 faunas, is so obvious as to require no argument. 



That the embryonic form of some Aplacophora possesses tempo- 

 rarily eight dorsal plates, indicates their place as extremely degen- 

 erate Chitonoids. It is my opinion that the characters allying the 

 Amphineura to the more evolved forms of Gastropoda have greater 

 taxonomic value than those relied upon to separate the former with 

 the rank of a class, in spite of the different decision arrived at by 

 some much respected authors. I have therefore restricted them to 

 the rank of subclass, assigning ordinal rank to the two groups Poly- 

 placophora and Aplacophora. I am indebted to Prof. Harold 

 Heath's discussion and classification of the west coast Aplacophora 

 for that part of the list. 



The material upon which the present list is based is almost exclu- 

 sively contained in the collection of the United States National 

 Museum and is the result of more than half a century of collecting, 

 beginning with the United States Exploring Expedition material 

 studied by Dr. A. A. Gould. Among the earlier collectors should 

 be named Dr. William Stimpson of the Ringgold and Rodgers ex- 

 ploring expedition; Major William Rich and Colonel Ezekiel Jewett 

 of the Mexican War naturalists; Caleb Kennerly of the Northwest- 

 ern Boundary Survey; the Rev. Joseph Rowell, an early California!) 

 missionary: William M. Gabb and J. G. Cooper of the Geological 

 Survey of California; Thomas Bridges. William P. Blake, and 

 R. E. C. Stearns. Of those mentioned most of the types of species 

 described by them, or by others from their collections, are pre- 

 served in the national collection. Of the collections made on the 

 Albatross voyages and by the officers of the United States Revenue 

 Marine and by the United States Bureau of Fisheries, beside my 

 own collections made from 1865 to the end of the nineteenth century 

 while in the service of the Western Union Telegraph Expedition, the 

 United States Coast and Geodetic Survey, and the United States 

 Geological Survey, the types are preserved in the same custody. 



