IN DESCRIBING SHELLS. 31 



Invobitus is when the spire is rolled in, so that both ends are 

 enveloped by the outer whorl, as in the genus Cyprcea. 



Convolutiis is when the spire is rolled in, so that one end only 

 is enveloped, as in the genera Conns, Voluta, &c. 



Rotundatiis is when the shell is rolled up in nearly the form of 

 a globe, as Bulla Naucum, Ampulla, &c, 



Sub-globosus is rather less globular, with the spire more ex- 

 tended, as Nerita Vitellus. 



Ovatus is egg-shaped, as Comes rnsticus and Mercator, 'Ne- 

 rita virginea, or Bulla fontinalis. 



Oblotig7is, or oblong, as Helix Amarula. 



Ovatus-ohloiigus is between egg-shaped and oblong, as Bucci- 

 num reticulatum. 



Pyriformis is when the shell is nearly pear-shaped, as in Voluta 

 Pyrum, and also one division of the genus Cotms of Lin- 

 nteus. 



Pyramidalis is like a pyramid, as Trochus cinereus. 



Conicus is conical, of which there are two sorts, one with the 

 spire flat, forming the base of the cone, as Conus marmo- 

 reus, and many of the genus Coims ; the other with the 

 aperture at the base, and the spire tapering, as Trochus ni- 

 loticus, and many species of Trochus. 



Cylindricus is nearly cylindrical, as Bulla Terebellum. 



Fusiformis is spindle-shaped, and differs from cylindrical in 

 being swelled out at the middle and tapering at each end, 

 as Strombusfusus. 



Turritus, towering, is when the spiral turns are numerous and 



