338 Natural History of Molluscous Animals: — 



What are vulgarly called Cornua Ammc>n/5 are fossil multi- 

 locular shells, and their preservation is so complete, that, from 

 these remains, we can form some tolerably plausible notions 

 of what has been the structure and habits of their animals. 



Further, the inmates of many univalves can close the aper- 

 ture of their shells with a horny or calcareous plug attached 

 to the dorsal part of the foot, and termed the operculum. 

 These are said to be operculated, while those which have no 

 protection of this sort are non-operculated shells. The for- 

 mer, in the opinion of some naturalists, are the links which 

 connect the univalves to bivalves, the operculum being con- 

 sidered analogous to the second valve, and Blainville indeed 

 goes so far as to call them sub-bivalves. * This is surely 

 straining analogies, and seeking them in things between which 

 there is no mutual resemblance ; but in fact the opinion is a 

 part of a theory relative to the mutation of one organ into 

 another, which has been brought forward and illustrated at 

 great length by some French physiologists, and too hastily 

 adopted by some of our own countrymen. We will probably 

 have a future opportunity of discussing this doctrine, which, 

 I may here remark, appears to be the offspring of metaphy- 

 sical subtlety, and as likely to lead to error in regard to the 

 functions of parts, as to guide us to a knowledge of their true 

 uses. 



To return from this digression, I remark that univalve 

 shells are again distinguished into such as have the margin of 

 the aperture entire, and into such as have it notched, channel- 

 led, or more or less lengthened into a canal. The latter are 

 said to be canaliculate, or emarginate, or effuse, and, as will 

 be afterwards shown, differ from the first in the habits of 

 their animals, so that the character is of greater importance 

 than is at first obvious. These distinctions and terms it is 

 necessary to remember, as I shall have frequent occasion to 

 use them in our future correspondence. 



However varied in external character, shells differ very 

 little in their chemical composition. They all consist of car- 

 bonate of lime united to a soft albuminous matter, and any 

 variation that occurs in different shells is merely in the rela- 



* Manuel de Malacologie et de Conchiliologie, p. 229. In another 

 place, however, he more correctly observes, " Quelques aiiteurs, et entre 

 autres Adanson, I'ont regardee comme I'analogue d'une des valves d'une 

 coquille bivalve, mais evidemment a tort ; car sa position, par rapport au 

 corps de I'animal, n'indique aucune analogic," &c. (p. 103.) [" Some 

 authors, and amongst others Adanson, have regarded it as the analogue of 

 one of the valves of a bivalve shell, but evidently wrongly ; for its position 

 with relation to the body of the animal does not indicate any analogy."] 



