42 University of Michigan 



lateral differs in much the same manner as does the central ; 

 like A. flagcUata, 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 (vmcalcified?) extending laterad from the 

 main, thickened portion. The outer tooth is also more slender 

 and the base is not so enlarged as in Hagellata. 



APEROSTOMIDAE 



Aperostonia 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 (ii.o 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 simnlacriun . 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 mcxicammi salleaniim (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 salleamim (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 



