X INTRODUCTION. 
and in time, by a repetition of the process, becomes of 
the same thickness with the other part of the shell. 
From the construction of Univalves it is evident, that 
this juxtaposition of successive layers can only take 
place at the aperture, and that in Bivalves the enlarge-= 
ment must proceed from the hinge round the circum- 
ference of each valve. 
The submarine existence of the larger portion of these 
animals precludes, in a great degree, the knowledge of 
their nature and habits, and is one of the causes that 
Conchology still remains obscured and confused by 
anomalies in many of its genera, while other branches 
of natural history, from being more easily investigated, 
are better understood. 
The character which appears to have guided Linnzeus 
in the formation of his genera, was, in Multivalves, the 
position of the valves; in Bivalves, the peculiarity of 
the hinge; and in Univalves, the exterior figure of the 
shell, the columella, the aperture, and form of the 
mouth. Hence every bivalve exhibiting an ovate hol- 
low in the hinge is termed an Ostrea; and in the Uni- 
valves a plaited columella constitutes the distinction of 
the genus Voluta, and an ovate aperture, terminating in 
a canal inclining to the right, distinguishes the genus 
Buccinum. But to this arrangement it is objected, 
that the inhabitants of shells of the same genus ought 
to be identical; and on this supposition, as it ap- 
pears impossible for the Ostrea malleus and Ostrea 
maxima to be the habitations of the same animal, 
they ought to be classed in different genera. Simi- 
lar inconsistencies will appear from a comparison 
