NOTES RELATIVE TO GAMBUSIA 181 
Thus, the Central and Southwestern material appears to be 
much more nearly related to each other than they are to the 
Eastern material. However, the Central material is sharply 
different from the Southwestern in the number of ossicles 
forming the terminal hook of the fourth posterior ray, since 
the Central material has usually 3 ossicles, while the South- 
western has usually six. The Central material in this respect 
resembles the Eastern material which has usually 2 ossicles 
in the terminal hook of the fourth posterior ray. 
In the number of ossicles of the fourth posterior ray which 
bear teeth on their posterior edge, the three forms show , 
marked and distinguishing differences. In the Eastern form, 
there are 7 ossicles (mean number,) which bear a single 
tooth. Both the Central and Southwestern forms lack these 
ossicles bearing two teeth. The Central material also differs 
sharply from the Eastern form in the fact that the teeth 
in the former are often straight and are set at a very sharp 
angle to the longitudinal axis of the ray (Figs. 11-13) while 
in the Eastern material the teeth of the fourth posterior 
ray have usually a curved outline, and their apices are directed 
either proximad or distad, (the apices of the proximal teeth 
being usually directed distad.) The angle which these teeth 
make with the longitudinal axis of the ray is very much more 
nearly a right angle. The Southwestern form differs from 
the Central form in making a less acute angle with the ray- 
axis, as well as in possessing usually more posteriorally- 
directed teeth on the fourth posterior ray. These are the 
main similarities and differences of structure of the gonopod 
in individuals of Gambusia from the three regions. 
It might be thought that the enumerated differences oc- 
curring between Central and Southwestern forms were due 
to differences in habitat. Difference in habitat will, however, 
account for very little of the variation found in the gonopods, 
for specimens from Maryland to Key West and from Illinois 
to Louisiana have a close adherence in all cases to the type 
characteristic of the Eastern and the Central form, re 
spectively, and the habitat of different regions in each of 
these two districts differs fully as much as does that of the 
