XX 
PREFACE. 
it affords for identifying relics of this kind. It is probable, 
however, that many of those shells now reputed species, will be 
found to be only varieties, by those who have it in their power 
to compare specimens from the same localities in different stages 
of growth. Perhaps not a few corrections are still necessary in 
regard to the physical distribution of the species. 
In quoting the various works in which have been illustrated 
the characters of the recent species, a reference has been made 
to those editions which represent the latest improvements of 
the authors. Thus the 12th edition of the Sy sterna Naturce 
of Linn^us is referred to, instead of the compilation by Gme- 
LiN, which not unfrequently supplies its place. A similar plan 
has been followed with the writings of Pennant, Pulteney, 
and others. By pursuing this plan, the author has avoided 
the somewhat awkward custom of quoting the authority of Lin- 
N^us and others for the names of species established subse- 
quent to their decease. The works which are referred to have 
likewise been quoted in a chronological order, for the purpose 
of pointing out the priority of the claims of the different obser- 
vers, and the grounds of the preference given to particular ge- 
neric or specific names. 
Though the author has undergone a very great degree of la- 
bour in the construction of the present work, he has much rea- 
son to fear that it will be found imperfect even in reference to 
published species. It would give him sincere pleasure to have 
either his mistakes or omissions pointed out, nor will he conceal 
his anxiety to obtain information respecting the discovery of 
new species. The great extent and variety of those publica- 
tions in which the discoveries of observers may be recorded, 
forbid any compiler, however industrious, to fancy that he has 
collected all the scattered documents of the science. The au- 
thor, however, has done his best, situate as he is at a great dis- 
tance from personal intercourse with zoologists, and opportuni- 
ties of consulting the journals of the day, and in a great mea- 
sure confined to an examination of those works which consti- 
tute his own limited library. He is aware that, by employing 
the common practice of copying synonimes, he might have 
concealed these imperfections. He has made a different choice, 
