Occasional Papers of the Museum of Zoology 55 
greatest diameter, 1.30 mm.; lesser diameter, 1.19 mm. ; height 
aperture, .47 mm.; width of aperture, .47 mm.; greatest diam- 
eter of umbilicus, .43 mm. 
This minute species appears from its shell characters to be 
most closely related to Thysanophora tater Pilsbry, and so 
belongs within the limits of the genus as at present constituted. 
It differs from that species by its much smaller size, by the 
gradually increasing whorls, and by its fine and regular striatu- 
lations. Under high magnification (700 diameters) all shells 
show some structure, which is perhaps caused by the edges of 
the crystals of which it is composed, but these are the most 
regular that I have ever examined. They are very much more 
regular and delicate than those of the Striatura-group of the 
Zonitidae. 
In order to differentiate this species more exactly, and to 
show its relations to the other Mexican and Central American 
forms, the following key is presented, which includes all of 
the species usually placed in Thysanophora, which have been 
listed from that district. I have examined under rather high 
magnification (at least 250 diameters) all of the forms included, 
with the exception of H. guatemalensis C. and F. and T. tur- 
binella (Mo.). ‘The position of these two forms in this key 
is doubtful, as their descriptions do not accurately describe 
the shell sculpture. 
A. Apical whorls spirally striate, without definite markings (Radio- 
discus?). 1.84; 63; 3%. coloba Pilsbry 
AA. Apical whorls with irregular punctations, somewhat radially 
arranged. Lower whorls also with irregular punctations, with 
1The numbers for each species indicate in the order given: first, 
the greatest diameter in millimeters; second, the height-index in per- 
centages, which is taken as the height divided by the greatest diameter ; 
and last the number of whorls. The greatest diameter is taken as a 
basis for the index, as this dimension is usually most accurately deter- 
mined in small species. 
