Occasional Papers of the Museum of Zoology 43 
specimens are yellowish horn-colored, shading into amber 
toward the tip. The growth-wrinkles are very regularly and 
evenly spaced. The following extremes show the variation in 
size of the adults: 
Altitude Greatest diameter Height aperture Diameter aperture 
Fig. 9 14.5 138 (20) 72 (10.5) 66 (10.5) 
Largest 16.5 152 (25) 64 (10.5) 61 (10.0) 
The jaw-plates of this species are practically the same as 
those of Aperostoma dyson (Pfr.). The radula (fig. 8) is 
also quite similar, but differs in several minor particulars. The 
central tooth in C. mexicanum is more elongate and the cusps 
tend to be somewhat more rounded than in the latter species. 
The outer cusp of the second lateral is almost vertical and 
faces inward. In the ordinary position of the tooth it appears 
as simply a blunt, vertical projection on the outer margin. of 
the tooth, but when seen in profile it appears more prominent 
than in 4. dyson, and projects out almost at right angles to 
the remainder of the cusps. The outer tooth has three cusps, 
as in A. dysont, but lacks the attenuate point in the lower cor- 
ner. In both A. dysoni and the present species there is not a 
definite base to this last tooth, as might be judged from Crosse 
and Fischer's figure, but the entire tooth forms a plate with 
three cusps on the inner side. The inner and central cusps 
curve inward and down, but the large, triangular, basal cusp 
faces directly inward. The tooth appears to be attached to 
the basal membrane by its outer edge. 
Cyrtotoma mexicanum mexicanum (Menke) (1830)—One 
adult (dead shell) from the burnt-over region (H, II, b), and 
6 adults and 2 young specimens from the strip of jungle along 
the upper portion of La Laja. These last woods are about 
intermediate in type between the lowland forests (H, I, a) 
and the savannah forests (H, III, a). 
This set of specimens presents evidence that mexicanum and 
