IV 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 Unioues, 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 Ostrece, 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 ' Index Testaceo- 

 logicus,' of which this work, written expressly as a com- 



