2 INTRODUCTION. 



Univalves (as the name denotes) are those shells which have only one external 

 covering ; such are the Whelk, Murex, or Conch kind. Bivalves are those which 

 are more complicated in their form, having two external coverings united by a 

 hinge, consisting of, or joined to, certain teeth, which are Various in number ; and 

 the form of these taken together constantly mark the distinguishing character of 

 each genus ; for instance, the Oyster (Ostrea) from the Muscle, or the Pecten or 

 Scallop from the Cardium or Cockle. 



Of the Univalve shells, the Murex, Plate I. is the first described, and, in order 

 to mark its peculiarities of character as distinctly as possible, and to distinguish it 

 more clearly from others which much resemble it, a Supplementary Plate, No.'LIV. 

 is added. Plate III. Monoplex, consists entirely of a genus lately discovered in the 

 Southern and Indian Seas ; it has a membranaceous ridge or projection placed 

 longitudinally upon the body. The Biplex has two of these, one on each side of 

 the shell, the Triplex three, the Hexaplex six, the Polyplex more than six and less 

 than twenty ; but the genus Septa differs from all these, by having the ridge placed 

 irregularly and alternately on the body and folds of the spire. 



In all the genera which follow the above, the forms of the mouth, the spire, and 

 the beak, are made collectively to point out the distinguishing characteristics of 

 each. Amongst these are several, which I have adopted from the ingenious and 

 acute observations of Messrs. Brugutere and Lamarck, authors of acknowledged 

 eminence in the delightful and instructive path of Natural History. Where Linnseus's 

 generic characters appeared to rest upon a solid and indubitable foundation, I have 

 adopted his names and definitions, though not without conceiving, that the genera 

 might have been made more numerous.* The genus Murex of the latter writer is for 

 the greater consistency, truth, and perspicuity, now altered, and divided into several 

 different genera; the Bulla, Voluta, Strombus, &c. have undergone the same necessary 

 extension and revision ; and the Helix, Natica, Pomacea, and Nerites, are divided 

 from each other for the first time, their difference of shape being peculiar and decisive. 



Of the Bivalves, the Ostrea is divided into two genera, Pecten and Ostrea, a change 

 which has long been a desideratum with the most eminent Conchologists. The 

 genus Pholas is included also amongst the Bivalves, from its great resemblance to them 

 in form, and its supplementary valves being considered as irregular membranaceous 

 appendages, and not always found to be attached to this curious shell. The Univalve 

 and Bivalve shells are found indiscriminately scattered over the various coasts of the 

 ocean, and the largest are to be met with only in the Torrid Zone ; but in the seas which 

 lie adjacent to the North and South Pole, the coldness of the temperature seems to 

 operate in reducing the size of animals, and of shell-fish particularly. The largest 

 shell at present known, is the Chama Gigantea, a Bivalve, about three feet in length, 

 one foot and a half in breadth, the shell itself being four or five inches thick. 



membranaceous animals, partly elastic, attached to other substances, by a flexible skin or base ; such is the Chiton, 

 the Balanus, the Lepas, and the Sabella. These certainly have neither the appearance or the conformation of 

 shell-fish, since, by the discoveries of Mr. Hatchett and other writers, it is found upon a chemical analysis, that 

 their coverings are not testaceous ; they also seem to form a gradation rather to the class denominated Vermes, 

 nor do they in general possess the power of loco-motion. The Pholas is classed with the Bivalves. 



* In the Index at the end of this work, the different genera adopted from the above writers are marked ; and 

 also those now introduced for the first time. 



