144 BULLETIN OF THE BUREAU OF FISHERIES. 



23 of these mollusks appear in the list of species which were taken at one-fourth or 

 more of the total number of stations. Thus exactly one-half of the latter list is consti- 

 tuted by Mollusca. 



We regard our molluscan records as being, on the whole, relatively complete and 

 comparatively free from error. The species are for the most part large and easy of 

 identification. Fortunately for the collectors, systematic conchology is based largely 

 upon shell characters, so that the determinations could commonly be made with a high 

 degree of confidence in the field. The few cases among the larger species in which 

 confusion was believed to be possible were early recognized, and we believe that errors 

 respecting such forms were nearly always avoided except at the beginning of the work. 

 Wherever doubt was felt, and especially in the case of the smaller species, specimens 

 were preserved for identification by specialists. We were fortunate enough to have 

 the assistance of such well-known authorities as Dr. W. H. Dall and Dr. Paul Bartsch 

 in the identification of the less familiar species of shell-bearing mollusks. We are like- 

 wise indebted to Dr. Dall for the critical examination of our manuscript check list 

 and for supplying us with the ranges of distribution which are given below. The classi- 

 fication and terminology adopted are his. a The nudibranch mollusks, on the other hand, 

 including many specimens taken in the townet, as well as those which were dredged, 

 were identified by Dr. F. M. MacFarland, of Stanford University, and Dr. MacFarland 

 has likewise kindly revised that portion of the manuscript devoted to this group. 



Certain sources of error have, notwithstanding, to be considered in the records for 

 the Mollusca. Some of the minute forms representing the genera Turbonilla, Odostomia, 

 Caecum, Cylichna, etc., were doubtless frequently overlooked, as likewise such small 

 species as Astyris lunata, Lacuna puteola, Triforis nigrocinctus , and Bittnim nigrum. 

 During the first season's work, especially when less thorough methods of sifting the 

 bottom deposits were employed, it is likely that the records for these forms were much 

 less complete than they were later. Again, the failure to use a canvas mud bag and 

 the consequent escape of the finer components of the bottom material doubtless resulted 

 in many cases in the loss of these small mollusks. 



As already mentioned, it was found that during the rather experimental earlier 

 work of the Survey certain forms having a close superficial resemblance had been con- 

 fused with one another. Since it is believed that these ambiguities have in most cases 

 been eliminated by special dredgings at the points in question, they can not seriously 

 affect the value of our results. Some of the smaller species of Natica {Polynices) were, 

 it is believed, wrongly identified in the field, and in such cases these records have been 

 entered merely as "Polynices sp." Even Polynices triseriata was not during the first 

 season always listed separately from Polynices heros, of which species it has often been 

 regarded as a variety or even as an immature stage. In consequence of this the records 

 for P. heros are doubtless somewhat fuller than they should be, those for P. triseriata being 

 correspondingly curtailed. 



In a few cases, notably with the small shells of the genus Turbonilla, confusion was 

 brought about by our failure to recognize the presence of several species among the 

 specimens taken. Instead of preserving samples of Turbonilla shells from every station 

 at which they were encountered it frequently happened that the collectors chose speci- 



1 Except that we have retained the Amphineura in a separate class. Dr. Dall has recently expressed the belief that they 

 constitute " at most an order." 



