178 THE AMERICAN MIDLAND NATURALIST. 
imperforate, it should be noted that the two sets of fin-rays 
in the fused median organ form on the anterior border of the 
gonopod a trough or furrow down which the sperm-ball, 
after being discharged, proceeds towards the genital pore. 
Copulation in these fish is only a momentary process, and 
is not 4n act of intromission. 
On studying the microscopic structure of the terminal 
portion of the gonopod of an ‘‘Eastern form” of Gambusia it 
is found that the simple ossicles of the anal fin of the male 
have become most curiously and characteristically modified. 
(Fig. 1). Three rays, the third, fourth, and fifth, form 
the terminal portion of this organ. The third ray forms 
an unbranched series of relatively thin, broad, conspicuous 
trough-plates, frequently having in the ‘Eastern form” 
prominent angular, lateral, longitudinal ridges. The most 
distal ossicles of this fin-ray are progressively smaller and 
possess hooks directed distad (‘‘serrae’ of REGAN.) The 
fourth ray is divided into branches in the proximal portion 
of the gonopod. The anterior branch has on its anterior 
edge a prominent, slightly recurved hook, at the distance of 
eight or ten ossicles from the end of the gonopod (Fig. 1, 
ant. hook, IV. ant.) The posterior branch of the fourth ray 
ends in a prominent terminal récurved hook (Fig. 1, term. 
hook, IV post.), to form which several ossicles may be fused, 
followed after a short interspace of simple ossicles by a 
number usually five or six, which have posteriorally-directed 
hooks or spines. The fifth anal ray is relatively simple, and 
is terminated by a retrorse hook. This, in brief, outlines the 
morphology of the terminal portion of the fin in the ‘Eastern 
form” of Gambusia. The ossicles, of course, are bound to- 
gether by connective tissue, and enclosed within the investing 
fin-membranes. 
The accompanying Figures 1-18 are camera lucida draw- 
ings of gonopods of specimens collected in the following lo- 
calities:. Goldsboro, N. C., (Figs. 1-2); Savannah, Ga., 
(Fig. 3) ; Augusta, Ga. (Fig. 4) ; Big Pine Key, Fla., (Fig. 5) ; 
Little Rock, Ark., (Figs. 6-7); Austin, Tex., (Figs 8-9) ; 
