VARIATION IN ARMATURE OF APERTURE. 
fiS 
that tliis plait has arisen hy their coalescence; in the same way the 
lainella-fnlcrans, which is placed npon the base of the pennltiinate 
whorl, would seem to be a result of the thickening of part of the 
lamella-spiralis, which blends with the similarly thickened lamella- 
inserta, till they spread across to neighbonring folds, for when the 
lamella-fulcrans is present, the lamella-spiralis and lamella-inserta are 
quite absent or so greatly reduced as to be scarcely perceptible. The 
superior lamella is sometimes joined to the lamella-spiralis, which 
latter, with the subcolumellar lamella, arises from near the point of 
attachment of the clansium to the columella. 
Fig. 117. Fig. IIS. Fig. 11!). Fig. 150. 
Meal iliayraiiunatie section.'^ of the shell of Cldusilia to show the position 
and arran^feinent of the prineipal lainelhe and idieations (diaracteristie of the 
G'einis (inodilied, after Smith and Woodward). 
Fig. 1 17. — Dorsal view, with outer shell-wall removed to show the lamelhe. 
Fig. 1 — Front view with portion of the shell-wall removed to show the position occupied by the 
clansium and the sub-columellar lamella, and also showing the folds visible exteriorly. 
Fig. 14!). — Front view with portion of shell-wall and columella removed to show the plica;. 
Fig. 150. — I3asal view with shell-wall removed to show the lamella;. 
a. Lamella superior. c. Lamella parallela. /. Plica; suturales. 
/>. Lamella; interlamellares. Lamella fulcrans. k. Plica principalis. 
c. Lamella inferior. Lamella spiralis. 1. Plicje palatales. 
(i 1. amelia subcolumellaris. h. Lamella inserta. m. Plica lunala or Lunella. 
b.s. Pasal or anterior sinus, cl. Clausium. c.f>. Callus palatalis. />.m. Plica; marginales. 
Tlie formation of tliese apertnral lamelhic or plications wliicli wind 
into tlie .shell are considered by Mr. W. II. Dali to be owing to the 
gradual contraction or narrowing of the later whorls, which necessarily 
throws into folds or wrinkles the conqjaratively volnminons mantle 
margin by compressing it between the muscular foot and the shell- 
wall. These ]»lications and lamelhe are formed by the semi-Hiiid 
seci’etion from the general surface of the mantle, and are moulded 
between tbe folds or wrinkles into which the mantle is thrown, the 
elevated wrinklings of the mantle pressing against the shell-wall 
