2234 Mollusks. 



discoidal form exhibited by some of the Patelliform species, in which the cone is so 

 depressed as to form an angle, from the vertex to the margin, of 170 degrees, to that 

 of an elongated tube, extended in some cases so much as to become nearly cylindrical, 

 and twisted into almost every conceivable form of spiral, for the convenience, 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 growtb, caused by a greater deve- 

 lopment 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 some- 

 times 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 cor- 

 responding 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 ani- 

 mal, 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 inti- 

 mate 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 dif- 

 ferences existing in the shells, and which variations in structure are generally consi- 

 dered 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 decomposition 

 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 accu- 

 rate 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. 



" In regard to the admeasurement of shells in the present work, minute fractional 

 descriptions have not been very particularly attended to, but a general approximation 

 only has been given, from a belief that a comparison of the dimensions of the aperture 

 and the axis of a shell is a test of fallacious dependence. In many instances, speci- 

 mens of the same species have a much greater range in variation than, as a general 

 character, would be exhibited between two proximate species ; the aperture in some 

 individuals being equal to two thirds of the length of axis, while in others of the same 

 species it does not exceed one third. In an elevated or cylindrical!}' formed shell, 



