46 University of Michigan 
until after counting the laterals. The fifth lateral is turned 
considerably inward, and is almost completely hidden by the 
distal end of the first marginal. The break to the marginals 
is a sharp one, and shows as a raised edge, even under low 
magnification. The well-developed marginals are all tricuspid 
and point obliquely inward, and the transverse row itself also 
slopes obliquely backward (1. e., in the direction towards which 
the cusps point). As the inner cusp of each marginal overlaps, 
_to a certain extent, the outer one of the preceding tooth, it is 
sometimes difficult to make out more than 2 cusps, which prob- 
ably accounts for Binney’s statement that only a portion of 
the marginals are tricuspid. The lenses in his time were con- 
siderably inferior to the modern oil-immersion objective. 
Amongst the outer reduced teeth, the thirtieth and thirty-first 
have four cusps each, while the outermost is a mere denticle, 
and is lacking in some of the transverse rows. 
For comparison, the jaw and radula (fig. 2) of Guppya 
sterkii (Dall)? was also examined. This species, as Vanatta 
(1920) has already pointed out, has a similar dentition to that 
of G. gundlachi, only the radular ribbon is so minute as not 
to fill the field of the oil-immersion objective. ‘The central 
tooth, for instance, is only about 4 microns (.004 mm.) in 
I 5 13-15 
width. The formula is approximately: C—;L—;M 
3 3 3 
The number of cusps out to the ninth marginal was determined, 
but their shape on this tooth could not be made out very accu- 
rately, as the ends of the cusps are smaller than the limit of 
possible microscopic vision, and so could only be detected as 
1 Dried animals; A. N. S. P. No. 46177; collected at the Clydesdale 
Brick and Stone Company Farm, Beaver County, near Elwood City, 
Paiby jee Clark 
