INTRODUCTION. ix 



venience, 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 shell, 

 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 sup- 

 position 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 decompo- 

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

 mechanical action of the waves. These accumulated difficulties are serious impedi- 

 ments 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. 



