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 Linnaeus 

 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 Voiuta, and an ovate aperture, terminating in 

 a canal inclining to the right, distinouishes the grenus 

 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 



