VARIATION IN ARMATURE OF APERTURE. 
07 
the aperture of the shell. Others, as densely deuticulate as Pupa 
secale, vary in the number of teeth possessed by the mature shell, 
and may even have a variety in which the whole of the denticles con- 
tracting the aperture have become obsolete. 
An interesting variety of Hijalln'm fulva, if the identification be 
correct, is recorded from Cincinnati, U.S.A., in which the immature 
shells develop within the interior of the last or body whorl, a number 
of little denticles, radiating from the umbilicus like the spokes of a 
wheel, appro.ximating in this respect to 
the character of Gastrodonta, of which 
genus several species are found in that 
region. This peculiarity, which dis- 
aiipears in the adnlt, is said to be a 
probably defensive provision against a 
small tender grub, which lives in beds 
of leaves and preys upon small mollusks. 
These structures are quite analogous 
with the denticles and folds found in the post-embryonal Piqvi 
cijlindracea and Pupa anglica, which also in a great measure disappear 
at maturity, and are thus a character especially distinguishing the 
juvenile stage of growth. 
In Claiisilia the mouth is furnished with a nnmber of very 
characteristic plaits or folds, which vary little in their relative 
position, and are therefore of specific and ta.xonomic importance, 
and e.xtensively relied upon by systematists as a basis upon winch 
to form subgeneric divisions. A full, complete and precise nomen- 
clature, and an accurate determination of tbe relative positions of 
the various plaits and folds was thus very desirable, and Messrs. 
E. A. Smith and B. B. Woodward, with the help of Dr. 0. Boettger, 
have proposed a terminology which is probably the most thorough 
and satisfactory yet published. These authors recommend the 
restriction of the term PLiciE'to the plaits situated upon the palatal 
or outer wall of the shell, and la.mell^e to those upon the columella 
and the columellar-wall above. One of the chief plaits is the plica 
lunata or lunella, a conspicuous and somewhat arcuate calcareous 
thickening upon the palatal wall, which is often visible through the 
shell, hut is however somewhat inconstant in form, being sometimes 
replaced by, or separated into, a series of very short plicm, ranging 
one above another in such a way as to suggest the strong probability 
Figs. 14o & liG.—Pu/n anglica 
CFer.) X 12, 
Roundstone, Galway, 
Collected by Mr. I’. Sturges Dodd. 
Side and liasal views of shell 
showing the internal folds or plaits, 
especially characterizing the j'outhful 
post-embryonal stage of growth. 
