282 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



sented in species and by larger individual species than anywhere else in 

 the world. That this is also true of other groups of Mollusca, such as 

 the I'lssiircllidw, Trochidce, Haliotidcc, Bucciniclce, and others, I hope here- 

 after io be able to show conclusively. 



Since I have elsewhere* treated in considerable detail the Limpets of 

 the northwest coast of America, 1 shall here present only a list of the 

 species with such additional material as six years' study and collections 

 have brought to hand, and reserve for the Chitons a more detailed ac- 

 count. This is the more desirable, since this group has been very gen- 

 erally neglected, and even, the most modern descriptions often fail to 

 give those details by which a species can be assigned a place among its 

 proper associates. 



Had the late Dr. Carpenter survived, the report on this group would 

 have been delegated to his more able hands ; the material passed for a 

 time into his possession, but his premature demise came to pass before 

 anything except the identification of the already known species and 

 some correspondence on the general subject had been accomplished. 

 For sufficient reasons, it is not to be hoped that his materials for a mono- 

 graph of the group, as a whole, will be published for some time, and I 

 have therefore been authorized to use some extracts from his MSS. 

 which have a direct bearing on the particular species here referred to. 

 I have in all cases followed him in framing descriptions of species, and 

 have quoted his original descriptions (giving due credit) where it was 

 practicable. Eesearch into several undecided questions has resulted in 

 decisions in several cases different from those he had anticipated ; but 

 in which conclusions, from my intercourse with him, I have no doubt he 

 would have eventually coincided, had he lived to follow out the investi- 

 gations he began. 



The caution, in assigning values to the higher divisions of this singu- 

 lar group, which was exercised by Dr. Carpenter, has been fully justified, 

 and it does not seem that our knowledge of them is yet sufficiently com- 

 plete to authorize definite conclusions. Examination of the radula, 

 heretofore almost wholly neglected, emphasizes the necessity of con- 

 tinued caution. The numerous characters presented by the insertion- 

 plates, the characters of the girdle, branchiae, sexual organs, develope- 

 ment, radula, and the presence or absence of pores on the upper sur- 

 face, are apparently interchangeable to a greater extent than would be 

 supposed. In this sense they present a remarkably homogeneous group. 

 In spite of numerous important and peculiar features, their position, as 

 a subdivision of the, Gasteropodous Mollusca, appears to me to be defi- 

 nitely settled beyond any reasonable question. By very numerous 

 characters, their continued association in the neighborhood of the Lim- 

 pets as their nearest (if still somewhat distant) relatives appears to me 

 to be assured, and requires only some knowledge of the embryology of 



* u On the Limpets," &c., Am. Journ. of Coucliology, vi, pp. 228-282, pi. 14-17, April, 

 1871. 



