42 University of Michigan 
lateral differs in much the same manner as does the central; 
like A. flagellata, three ectoconic cusps are often present. The 
second lateral (or first marginal) is more slender, is ligulate 
in shape, and is not double at the base, although there is a 
thinner portion (uncalcified?) extending laterad from the 
main, thickened portion. The outer tooth is also more slender 
and the base is not so enlarged as in flagellata. 
APEROSTOMIDAE 
Aperosioma dysoni (Pfeiffer) —Eighteen specimens: from 
the burnt-over area (H, II, b, dead shells) ; and from leaf- 
humus in the lowland jungles (H, I,a). The largest specimen 
measures: altitude, 15.8 mm.; greatest diameter, 129 (19.1 
mm.) ; height aperture, 69 (11.0 mm.) ; diameter aperture, 65 
(10.2 mm.). 
The radula and jaw-plates of this species were examined; 
they have been figured by Crosse and Fischer. (1888, 1890). 
They also figure what they term “elements” in both this species 
and in Tomocyclus simulacrum. In my specimens, these “ele- 
ments” look as if they were the cells, or that each one was 
the product of a single cell, and they are not regular in size 
throughout the plate. ‘Toward the edge they are longer, and 
are lanceolate to long trapezoidal in shape, while toward the 
center they are more nearly square or, more often, polygonal. 
The arrangement of these elements causes the apparent stria- 
tions, seen under low magnification; this loses its regularity 
when examined closely. 
Cyrtotoma mexicanum salleanum (von Martens) (1865 ).— 
Eleven adults and 6 young shells from under leaves in humus 
in the lowland forests (H, I,a). These shells are quite typical 
of salleanum (fig. 9), which apparently is the more general 
form of the species in the favorable, damp habitats, as will 
be discussed more fully under the form mexicanum. These 
