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 Aperostouia dysoni (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 A. dysoni, and projects out almost at right angles to 

 the remainder of the cusps. The outer tooth has three cusps, 

 as in A. dysoni, 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 mcxicanuui ine.vicaniiui (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 



