THE NATURAL HISTORY SURVEY. 39 
In the gastropods (univalves) orsnails the shell is in but one 
piece and is in the form of a spiral, except in Ancylus and 
Limax. In a specimen of Polygyra albolabris the following 
characters will be noticed (Fig. 4): The evenly wound, closely 
PoLYGYRA ALBOLABRIS, showing parts of shell (after Binney, L and F-W, 
Sh; Figs. 227, 229). a, apex; ap, aperture; d, denticle; lw, last whorl; p, 
peristome; pw, parietal wall; s, suture; sp, spire; u, umbilicus. 
coiled whorls; the small, light horn colored apex (a); the deeply 
impressed sutures (s); the large and swollen last whorl (lw); the 
wide aperture (ap); the white, reflected peristome’ (p) and the 
umbilicus (u) which-is wide and deep in the young shell, but 
covered by the reflected peristome in the adult. The shell is 
also seen to be covered with rather coarse growth lines and also 
fine lines running parallel to the whorls. The parietal wall 
(pw) is covered with a spreading callus and has frequently a 
well-developed tooth or denticle upon it, situated near the upper 
part of the reflected peristome. In the gastropods, as in the 
pelecypods, the shell is a protection to the soft parts of the 
animal, which is attached to it only by the columella muscle. 
The structure of the shell substance is essentially the same as 
that described under the pelecypoda, except that the inner layer 
is not nacreous, and the outer layer, or epidermis, is not so 
heavy, except in some of the fresh-water forms. 
As in the pelecypods, the shell is secreted by the mantle 
and conforms closely to the shape of that organ. The spiral 
shells are nearly mathematically correct, as much so as a living 
body can be. Most gastropods, excepting Pulmonata, are pro- 
