1V PREFACE. 
relative to the extent of permitted variation and the priority 
of appellations, prevailed among the naturalists of America, 
that it appeared advisable to adopt, as a whole, the conclu- 
sions of one who has devoted a lifetime to the almost exclu- 
sive study of fresh-water Testacea. This portion will be 
found extremely useful to the lovers of Uniones, as it gives 
the proper references to the works of those writers, whose 
names only had been quoted in the ‘ Synopsis,’ and, more- 
over, furnishes condensed descriptions (compiled from the 
volumes indicated in the synonymy) of almost all the 
Naiades known at the period when it issued from the 
press. 
A work written at long intervals of time (from 1842 to 
1856) must of necessity prove unequal in its execution: 
were the earlier pages reprinted a great many errors would 
be avoided; they sprung chiefly from a modest deference to 
the opinions of older conchologists. ‘The earlier published 
descriptions were, in the main, either translations, copies 
or abridgements; those of the Appendix, on the contrary, 
have been almost invariably drawn up from the shells them- 
selves and frequently from the original types. Peculiar 
labour has been bestowed upon the Ostree, Spondyli and 
Mytilacea ; Terebratula, likewise, has been worked up with 
more than ordinary attention. 
Several of the figures are only miniature copies of pub- 
lished engravings, but the cabinets of Cuming, Metcalfe 
and Hanley have furnished the artist with subjects for the 
majority of the illustrations. Due care has been exercised 
in avoiding the delineation of species which had already 
been adequately represented in the ‘ InpDEx TrEsracro- 
Locicus, of which this work, written expressly as a com- 
