INTRODUCTION. 3 



In the delineations of all the Univalve shells, I have adopted that manner of 

 viewing them, which is most favourable for shewing the character of the mouth, 

 the part, of all others, of principal use in determining and ascertaining the genera. 

 Within the mouth is- always placed the columella, or little central pillar, which 

 is joined to the beak, or opening at the baseband forms by continuation, a screw 

 in the centre of the spire, closely covered up by the circular folds of the summit. In 

 the Murex, the columella is only slightly fluted, or tending to that form ; but in 

 the Voluta, Volutella, and Buccinella, it is in form a screw very strongly marked ; 

 in the Pleurotoma it is quite smooth. In short, the character of each genus is chiefly 

 determinable by its exterior form, and the species in a great measure fixed by the 

 size and the colour of the shell. In the Patella, the circumstance of the difference 

 of colour is particularly remarkable, and determines, along with the roughness or 

 smoothness of the covering, the specific character to which it is naturally allied. 

 The genus Argonauta has its volute centrally involved upon itself, and ending rather 

 abruptly in the rounded middle, in this respect differing from the Planorbis, in which 

 the curvature is always raised externally. 



The genus Columna, although it considerably resembles the Helix, has characters 

 peculiarly its own; for instance, its lengthened form and the foldings of the spire, 

 revolving the opposite way to most others in nature, constitute what is denominated 

 a Heterostrophe shell ; in this respect it seems to be unvaried, yet other genera have 

 sometimes, from accident, this singular direction. The Trochus is distinguished 

 from the Turbo, chiefly by the square character of the mouth, and from its position 

 being underneath the shell, whereas in the other it projects in front. The genus 

 Bulla is clearly distinguishable from the Cymbium, by not having its spire standing 

 out, but reverted into the cavity or body, in a way something similar to the Cyprsea, 

 which has the cheek, or maxilla oris, inserted in a like manner. 



Of the Bivalve shells, different perspective views have been given of their forms, 

 for the purpose of explaining those irregular and cycloidal curves which frequently 

 occur in this tribe; the same shell being often flattened in one direction, and 

 circular in another ; the character of the genera is taken entirely from the form of 

 the hinge and the teeth, which it is much easier to describe than to delineate ; but 

 the circumstance of some of the Bivalves being wholly without teeth, and having only 

 an external hinge, leaves this part of the subject in less certainty, than in the Univalve 

 division. To remove this serious difficulty, some modern writers have proposed to 

 class the Bivalves by genera drawn from their external forms, and it is not impossible, 

 but at some future period the additional discoveries of new species or genera, par- 

 ticularly of those in a fossil state, may throw an additional light upon the subject, 

 and be the ultimate cause of a new Methodus Generum of this part of the science ; 

 for, as our knowledge of Natural History is progressive, it is certain that all descrip- 

 tions or systems must be progressive likewise, subject to be enlarged and amended, 

 rescinded and altered, in proportion as that knowledge becomes extended. 



The difficulties of the science of Conchology seem to have arisen from the extensive 

 number of different forms, and at the same time the awkward and imperfect delinea- 

 tions which have generally accompanied works written on this subject. The multitu- 

 dinous Plates of Lister, Martini, Pennant, and Da Costa, taking either a wider or 

 more confined range through this difficult, yet precise branch of natural History, 

 impart only to the reader representations ill executed, and descriptions worse 



