INTRODUCTION. 



clearer. Dr. Jackson's investigations' led him to the con- 

 clusion that characters almost inseparable from those of the 

 adult frequently make their appearance in the brephic stage of 

 the moUusca. In view of these facts it seems desirable to modify 

 the general statement that during the brephic stage no specific 

 characters can be distinguished. As a rule, no doubt, that statement 

 is perfectly accurate, and the brephic stage is unquestionably well 

 defined in the Gasteropoda as a whole ; but a little qualification 

 seems necessary. I have found that the greatest difficulty in 

 defining the brephic stage is in those cases where a strong varix 

 has been thrown up at the conclusion of the embryonic stage, and 

 it seems right that that should be so. Por the varix certainly 

 indicates a pause in the growth of the shell, and it is reasonable 

 to assume that during that pause the animal was passing through 

 the brephic stage, but did not continue to make the normal shell 

 of that period, except partially, it may be, in some instances. 

 I have even doubted whether in certain instances the varix 

 alluded to was not, in fact, the only manifestation of the growth 

 of the shell during the brephic stage. 



Turning now to some other points brought out in this Catalogue, 

 it may be noted that it introduces frequent changes in nomen- 

 clature, especially in regard to genera and subgenera. Many 

 well-known names, I am sorry to say, have disappeared as the 

 result of bibliographic research. But I am not proud of having 

 made these changes — they were inevitable if the rule of priority 

 must be applied. 



In particular, I have been at some pains to discover the names of 

 the type species of both genera and subgenera, in consequence of 

 which a new interpretation has, occasionally, had to be placed 

 on a well-known group. Some new genera and species are 

 proposed; and species hitherto described have been considerably 

 revised in regard to their generic position. In the description of 

 species my general plan has been to give only the salient features, 

 usually in the nature of a short diagnosis, where the species had 

 formerly been fully treated of ; but where the material in the 

 Museum collection permitted, and the species had hitherto received 

 but scanty notice, it has been dealt with at greater length, 



' "Phylogeny of the Pelecypoda " : Mem. Boston Soc. Nat. Hist. vol. iv. 

 No. 8, 1890, p. 290. 



