UNIVALVES. 



PLATE III. 



Genus. MONOPLEX. 

 Character. Shell univalve, spiral, having one fold, or membranaceous division, 

 placed longitudinally upon the body, from hence the name Monoplex (or of one 

 fold). The division is not continued farther than the first revolution of the spire, 

 nor is it again repeated, as in the Biplex, Triplex, &c. The cheek of the mouth is 

 labiated, and furrowed. 



Species. 



No. 1. Monoplex cornutus. Shell brown, with a parti-colour division; the 

 spire invested with small horns projecting ; the mouth brown. This curious 

 shell has been lately introduced from New Holland, and is now pretty 

 common. 



No. 2. Monoplex obgesus. Shell brown, flattened at the top ; the beak long and 

 taper ; the mouth wrinkled and narrow. A native of the East Indies, and 

 rather rare. 



No. 3. Monoplex Australasia. Shell of a rich brown, variegated on the 

 mouth with black and white streaks ; the spire formed at the top into ring- 

 shaped folds. Lately introduced from New Holland, and Lord Howe's 

 Island. 



No. 4. Monoplex capitatus. Shell brown, slightly beaded ; the mouth convo- 

 luted ; the apex very conspicuous, in the shape of a ball ; the rostrum 

 very long. Native place unknown. 



No. 5. Monoplex formosus. Shell of a pink colour, of exquisite symmetry, and 

 pleasing form; the body and mouth gracefully undulated with irregular 

 bands ; the mouth pale red or white. Native place, Ceylon and the East 

 Indies. 



REMARKS. 



The genus Monoplex is now for the first time distinguished from the Murex, and indeed 

 from all other shells known, having one membranaceous division, which reaches only to 

 the first revolution of the spire, forming a sort of projecting ridge, which may be compared 

 to that of the Biplex, Triplex, Hexaplex, Polyplex, and Septa, all which are classed ac- 

 cording to the difference of number and arrangement of the folds. It will be found that the 

 Monoplex has no membranaceous continuation of the folds in the spire, which all the above 

 named genera have. A careful examination of the natural distinctions, either in the Speci- 

 mens or in the Plates, will at once serve to distinguish, and determine the appropriate cha- 

 racter of each. Add to this, that there are certain natural differences existing in these oppo- 

 site genera, which add to and confirm the more striking characters, and in which, as in bo- 

 tany, the reader must be referred to the objects themselves, since they elude all other deter- 

 mination. Thus the Biplex has a character of flatness, which no other shell has, as if it had 

 been pressed between two weights ; and similar peculiarities frequently occur, in which we 

 must be guided solely by the general and defined character adopted from the form and 

 structure. 



