IN DESCRIBING SHELLS. 39 



Posticum is when the partition is at the back part of the inside, 

 as in Patella Porcellana. 



OF THE OPERCULUM. 



Linnaeus scarcely mentions any of the opercula : Rumpliius 

 and Guallieri have given figures of several of them ; most of 

 the Turbines have thick testaceous opercula exactly closing the 

 aperture, and so have most of the Neritfe ; but these are not in 

 general quite so thick as those of the Turbin€s,hut they also take 

 the shape of the mouth of their respective shells ; very fevv^ of 

 the Helices have an) , except Helix vivipara and tentaculata ; 

 those are thin and horny, and exactly fit the aperture. Several 

 Buccina, Murices, and Coni, &c, have also opercula, but these 

 extend in size to only a part of the aperture. 



OP THE SURFACE OF SHELLS, SUPERFICIES. 



Leevis is when the surface is smooth and even, as in Mactra 



stultorum and Valuta porphyria. 

 Licevigata is as if worn or washed smooth, as in Tellina Icevi- 



gata and Buccinum decussatum. 

 Glabra is with the outside bright, as Venus Meretrix and 



Conus Capitaneus. 

 Glaberrima is very bright, as Venus castrensis and Buccinum 



glabratum. 

 Nitida is shining, as Tellina radiata and Turbo petholatus. 

 Uneatis is marked with coloured lines, as in Conus Princeps. 



