INTRODUCTION. ix 



or rather according to the necessities of the animal. Among the spiral shells these 

 varying forms are the result of an obliquity in the mode of growth, caused by a greater 

 development or increase of one side of the opening, whereby an heliciform direction 

 is given to the shell in the process of enlargement. It is, perhaps, needless to say, 

 that the mode of increase or growth of the shell is by the successive deposition of 

 calcareous matter secreted by the mantle, and deposited on the margin of the aperture, 

 and moulded as it were upon the soft body of the animal, by which means the impress 

 of every peculiarity is formed, and permanently fixed in the calcareous covering. It 

 may, however, be observed, that some of the exterior ornaments of these shells are 

 formed by an extension or protrusion of the mantle, whereby a fimbriated, and 

 sometimes a spinose, varix is formed upon the margin of the outer lip, producing 

 many varied and beautiful ornaments upon the spiral sheil, which, however, 

 have no corresponding characters upon the soft body of the animal. It is from 

 these appearances alone, that presumed specific distinctions can be determined ; and 

 the characters drawn from them, although they may be considered as somewhat 

 extrinsic to the animal, are the only means afforded to the palaeontologist for the 

 determination of species. From the recent researches of Mr. Bowerbank and 

 Dr. Carpenter, in their Microscopic Examinations into the Structure of Shells, it would 

 appear that there is a more intimate connexion between the animal and its envelope 

 during its lifetime than could be imagined by the supposition of a simple addition of 

 earthy matter deposited upon its cuticle ; and that this strong covering is an organized 

 body, and part and parcel of the animal itself. Important as are those observations 

 and suggestions, which bear considerably on generic groups, and by which, perhaps, 

 even minor distinctions may hereafter be pointed out, the specific determinations 

 referred to in the present monograph have not been submitted to such tests, but are 

 merely drawn from the various external differences existing in the shells, and which 

 variations in structure are generally considered as resulting from some internal economy 

 of the animal. 



This mode of determination may, perhaps, be correctly applicable only to shells in 

 perfect condition ; whereas, some of those figured in this work have undergone con- 

 siderable alteration, either by the loss of colour and animal matter, or by decom- 

 position of the outer coating, while many of them have sustained mutilation by the 

 mechanical action of the waves. These accumulated difficulties are serious impediments 

 to accurate comparisons ; and the identifications in many cases must be looked upon 

 only as approximations : a correct Marine Fauna of this period will, it is to be feared, 

 be for many years to come a desideratum. 



