44 University of Michigan 
salleanum are ecological growth-forms of the same thing. 
According to von Martens (1890), the only trustworthy dis- 
tinctions between the two are the larger size of salleanwm and 
the peristome. He describes the latter as follows: “lower lobe 
of the columellar margin beneath the deep notch is always free 
in C. mexicanum and soldered to the penultimate whorl in C. 
salleanum; this seems to be a constant character.” 
The newly formed peristome of this species 1s smooth on 
its outer surface and is usually regularly attached to the body 
whorl, although some specimens (for example, F. and C., /. c., 
pl. xxxv, 4; also’ Specimens in) the (A. NESS) E>) apparently; 
have a slight scalariform tendency. Under the most equable 
conditions of the environment, this condition of the peristome 
is apparently retained; so that in the lowland forests all of the 
specimens are quite typical of salleanum (fig. 9). However, 
the size cannot be used as a specific character; although the 
specimens of salleanum tend to be somewhat larger, the small- 
est specimen obtained (20 mm. in diameter) belongs to this 
form. 
The differentiation of C. mexicanum mexicanum from this 
type is apparent in the specimens from along La Laja. In 
these drier habitats there appears to exist a tendency to pro- 
duce the reflected peristome when smaller in size (younger ?). 
Probably on acount of the repeated periods of aestivation, 
additional layers of material are secreted over the outside of 
the peristome, as shown in the figures (figs. 10, 11 and 12). 
These additional layers are most extensive on the palatal and 
basal portions and tend to widen the peristome as well as 
increase its thickness (fig. 10). On the columellar margin, 
the added layers fail to attach themselves to the penultimate 
whorl (fig. 11), so that finally the typical mexicanum is formed 
(fig. 12), in which the lower lobe of the columellar margin is 
