TERMINOLOGY. ¥ts 
4. Acuminated, (acuminatum), when the upper 
part is drawn out into a very long point, fig. 137. 
5. Flat, (planum), when the operculum is quite 
flat. 
6. Mucronate, (mucronatum), when the operculum 
is quite flat, but on the upper side, in the centre, has 
a bristle-like point. 
C. The Fringe, (fimbria s. annulus), is a nar- 
row sinuated membrane, that is set with small 
membranaceous teeth, and lies within the 
operculum. ‘This body possesses great elas- 
ticity, and thus serves to throw off the oper- 
culum from the theca, fig. 261. 
D. The Mouth, (peristoma s. peristomium), is 
the membranaceous rim which surrounds the 
mouth of the theca. ‘The peristoma is of 
two kinds: 
1. Naked, (nudum), that is intire without either 
teeth or eminences, fig. 178. 
2. Figured, (figuratum), set with membranaceous 
teeth. 3 
a. With one row, (ordine simplici dentatum), 
when there is a single row of teeth round 
the opening. These are distinguished accord- 
ing to their number and situation, &c. as, 
é. Four, sixteen, or thirty-two dentated, 
(quadri, sedecim, vel 32 dentatum). No other 
differences in the teeth have been yet ob- 
served, fig. 176,177, 179, 180. 
&, With divided teeth, (dentes bifidi), when 
the points of the teeth are divided. - 
y» Twisted, (contorti), when the tecth are 
Hi drawn 
