UNIVALVES. 



PLATE LI. 



Genus. TRIGONA. 

 Character. Shell spiral, the folds of the spire triangularly formed and tubercu- 

 lous ; the mouth triangular, having immediately behind it a large umbilicus, which 

 reaches internally to the top of the shell, forming a complete opening. 



Species. 



No. 1. and 2. Trigona pellucida. Shell white, inclining to a pale brown, tuber- 

 culous ; the mouth triangular, of a different form from the mouths of all 

 other shells at present known, and also from the Trochus family, which are 

 generally square-shaped in the mouth. This most singular and unique shell 

 is from the South Seas, and in the possession of Miss Mitford, to whose 

 Museum it forms a striking appendage. 



Genus. PLAN ORB IS. 



Character. Shell circular, flattened ; opening of the mouth depressed, marginated, 

 and partly spread out, or patulous. The Planorbis resembles by its general form 

 the Ionic volute in architecture. 

 No. 3. Planorbis divaricatus. Shell of a pale amber colour, richly mottled with 



red veins and clouds ; mouth expanded, all the folds of the shell compressed 



together, and hollow on the under side. From a specimen in the British 



Museum. 

 No. 4. Planorbis dilatus. Shell white, with variegated bands of red and brown ; 



mouth expanded, arcuated. From a specimen in the Collection of Miss 



Mitford. 

 No. 5. Planorbis collapsus. Shell olive-coloured, having bands of blue and red; 



the mouth garnished with a green band, the whole shell very flat. 



Genus. COLUMNA. 

 Character. Shell spiral, elongated in the spire, the top rounded and blunt in its 

 form ; mouth very narrow, and pointed at the top ; no beak, but the base rounded, 

 and joined to the cheek ; the folds swelling and irregular in their size and forms. 

 No. 1. Columna grisea. Shell gray, streaked with brown; mouth pointed. A 

 native of Barbadoes and Jamaica. From a specimen in Mr. Radstock's 

 Museum. 

 No. 2. Columna marmorea. Shell of an amber colour ; the mouth gray. 



REMARKS. 



The singular genus Trigona, is now delineated for the first time, and in respect to its mouth, 

 forms a curious exception to all other shells at present known. 



The Planorbis is delineated in two different ways, to illustrate its form more clearly, No. 3 

 shewing it in a direct manner from the top, and No. 4 shewing the mouth-view. 



The two shells of the Columna genus, given above, are of that kind which are called by 

 Naturalists, heterostophus, that is to say, the folds are twisted in an opposite direction to 

 most other shells, forming thereby a curious exception in Nature, which seems to be a 

 constant occurrence in this genus. 



