182 THE AMERICAN MIDLAND NATURALIST. 
other two districts. No cases of overlapping were found 
between Central and Southwestern forms, or between Eastern 
and Central forms. 
All of the individuals studied were Gambusia. (REGAN, 713, 
Figs. 168-169.) There is no possibility of males of other 
genera having without our knowledge gotten into our col- 
lections, for the microscopic structure of the gonopod in the 
other viviparous Poeciliids living in the United States are 
sharply dissimilar from it in form. The merest tyro could 
instantly -distinguish them. 
The Gambusia type which I have here called the “EKastern 
form” corresponds in its structure with the species figured 
by REGAN (Fig. 169, A) as Gambusia holbrooki. None of his 
figured gonopods of other species of Gambusia resemble in a 
close way the structure of what I have called the “Central” 
and “Southwestern” forms of Gambusia. In Gambusia 
nicaraguensis (Fig. 168, A), G. wrayi (Fig. 168, B), and 
G. oligosticta (Fig 169, B), there are sniall points of re- 
semblance to our material, but with these points of re- 
semblance there exist fundamental and glaring differences. 
The Eastern form of Gambusia is the only one of the group 
in the United States which possesses denticulations on the 
posterior border of the ossicles of the third ray. In all other 
species, so far as I am aware, these borders are entire in out- 
line. 
The distribution of the Eastern form is from New Jersey to 
Key West, and it has invaded to a slight extent the Gulf drain- 
age of western Georgia, being found in the Flint R. at Cam- 
milla, Ga. The exact limits of its East-West range are, in the 
lack of material, not yet ascertained. 
Practically all students of the Poeciliidae agree with REGAN 
(713) in the conclusion that the “differences in structure of 
the intromittent organ are of great systematic importance.” 
EIGENMANN (’07, 712), LANGER (713), and HENN (’16) have 
used them as a basis for ascertaining generic relationships and 
affiliations. These differences of structure have proved to be 
remarkably constant to a type. Species within the same genus 
(as, e. g., the various species of Gambusia whose gonopods 
