EEPOET ON TPIE SCAPHOPODA AND GASTEEOPODA. iii 



reports on these subjects are already published. The present work has a short Appendix 

 on the Caecidae, from the pen and pencil of the Marquis de Folin. The remaining groups 

 of the Mollusca have been placed in the hands of other naturalists for description. 



On one or two points of this Report it may be well that a few words should 

 be said. 



The standard of authority to which in the main all the previously known species have 

 here been referred, and by which they have been determined, is the British Museum collec- 

 tion. A good deal of careful work was also done in the museums of Paris, of Geneva, 

 and of Berne ; but where the British Museum possesses specimens of the species under 

 examination, it is to these specimens that the Challenger shells have been brought. So 

 much I feel it needful to say, lest any one should think that determinations based on the 

 identifications of a private museum are not very trustworthy. That my results are 

 always right it would be vain to expect, still less that they will always be thought so. 

 Re-examination has checked some errors, but there are doubtless others, for which I ask 

 forgiveness beforehand. For some dislocation in the order of the figures in the plates 

 an explanation is due. The arrangement is not all that could be wished, but in several 

 cases specimens even in considerable numbers reached me long out of date, delayed in 

 the hands of the naturalists engaged on other portions of the Zoological collection — a 

 delay largely inevitable from the employment of Molluscan shells by Crustaceans and 

 other animals as their home. The somewhat heterogeneous arrangement of the large 

 group of the Pleurotomacea resulted from circumstances entailing protracted absence 

 from home, and making careful supervision of that part of the work impossible. 



In quoting authorities in connection with known species, I have not attempted to present 

 a full list of synonyms, or an exhaustive catalogue of authors such as is to be looked for 

 in the monograph of a particular group. The leading idea suggested by Sir Wyville Thom- 

 son from the first is what I have kept in view, viz., to mention synonyms where these are 

 for any reason important, and to refer to the leading authors, British and foreign, to 

 which any one at home or abroad would naturally turn in order to ascertain what any 

 individual species named may be. In cases where species have been already figured, a 

 reference to such figures has been given ; in a few cases of old species where no figure has 

 been given previously, or the figure supplied is not very accessible, a figure will be 

 found here. On Plate XXVII. a figure has been given of Xenophora pallidula, Reeve, as 

 exhibiting the very striking result in this case of the generic habit possessed by the animal 

 of attaching foreign bodies to its shell. 



It will be observed that the arrangement I have adopted is not alphabetical. That 

 system has many advantages ; but, in spite of some honourable examples of its adoption, 

 has not found general acceptance. 



The system of classification followed is, then, in the main that of the Messrs Adams, 

 and their work on the Genera of Mollusca has -been adopted simply as that substan- 



