44 University of Michigan 



salleamim 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 salleanmn and 

 the peristome. He describes the latter as follows : "lower lobe 

 of the columellar margin beneath the deep notch is always free 

 in C. mexicanuin and soldered to the penultimate whorl in C. 

 salleamim; this seems to be a constant character." 



The newly formed peristome of this species is smooth on 

 its outer surface and is usually regularly attached to the body 

 whorl, although some specimens (for example, F. and C, /. c, 

 pi. XXXV, 4; also specimens in the A. N. S. P.) 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 salleamim 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 



