176 THE AMERICAN MIDLAND NATURALIST. 
regard those of the United States as comprising a single 
species, Gambusia affinis and they look upon G. holbrooki and 
G. patruelis (along with other names long since gone into the 
synonymy) merely as varieties, or as entirely synonymous 
and without taxonomic standing. 
In the course of studies on top-minnows the writer has 
found evidence that we have in these forms certain well- 
defined types. This evidence is here presented. It is drawn 
from the microscopic structure of the terminal portion of the 
copulatory organ, or gonopod, of the fish. 
It will not be necessary to speak here of the general form and 
structure of the gonopod, and its origin, development, and 
microscopic structure in the different genera of Poeciliid 
fishes, for this has already been done in the admirable papers 
by LANGER (’13), REGAN (’18), MEEK & HILDEBRAND (’16), 
and HENN (16), and by EIGENMANN (’07, 712). Suffice it to 
say that in the viviparous Poeciliids the anal fin is meta- 
morphosed in the male into a complex, usually imperforate 
copulatory organ or gonopod, by means of which the male is 
able to transfer balls of sperm, the specialized spermatozeug- 
mata (sing., spermatozeugma) from his own genital aperture 
to that of the female. The third, fourth and fifth anal-fin 
rays are modified in male Gambusia to form the distal portion 
Text-figure A. (a) Lateral dissection of a male Gambusia holbrooki, 
showing the general relations of gonopod to other organs, especially 
the internal organs of the body. (J, intestine; T, testis, AF’, gonopod; 
HS, modified haemal spines; M, muscle controlling gonopod; VP, ventral 
process of abdominal vertebrae.) (b) Detail drawing of end of the 
gonopod. (After KUNTZ, ’14.) 
