NOKTHWEST COAST MARINE SHELLBEARING MOLLUSKS. 7 
culiarity of situs, as in Crejndula^ 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 centuiy 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 Einggold and Rodgers ex- 
ploring expedition; Major IVilliam Rich and Colonel Ezekiel Jewett 
of the Mexican War naturalists; Caleb Kennerly of the Northwest- 
ern Boundary Survey; the Rev. Joseph Rowell, an early Californian 
missionary; William M. Gabb and J. G. Cooper of the Geological 
Survey o:^ 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. 
