PREFACE. 
VUVADAARUAAVAYD 
Tus Manual, like those of the Mammalia and the Orni- 
thology already published, originated in the notes and 
drawings collected during a study of the subject, prepa- 
ratory to a second travel in Africa. I had intended to 
arrange the similar materials for the publication of the 
Reptiles and Fish before the Mollusca, until a further 
progress in the study of Geology convinced me that the 
present Manual was not wanting to the Zoologist alone. 
M. Brongniart has shown that the Mormations containing 
the remains of marine shells, very rarely, and then under 
peculiar and evident local causes, present those of fresh- 
water shells ; and that the contrary had been alleged. be- 
cause Genera containing both marine and fresh-water 
shells, differing not merely in specific but in generic cha- 
racters, had nat been reformed or subdivided. M. Lamarek 
acknowledged the justice of the observation, and resumed 
his labours, which had previously advanced this branch of 
Natural History considerably towards perfection, by es- 
tablishing separations of marine, fresh-water, and ter- 
restrial shells; but, unfortunately, his Laxtrait du Cours 
de Zoologie, published in 1812, contains the mere outline 
of his system, without the descriptions of the genera; and 
the Ist and only part (which appeared two years ago) of 
the 6th volume of his Histotre Naturelle des Animaux 
sans Vertebres, without figures, contains merely the three 
or four first genera of the Univalves, and reminds us so 
frequently of the melancholy affliction which has now 
compelled this illustrious Naturalist to depend entirely on 
the eye-sight of others, that the appearance of the remain- 
