FISHES FROM MEXICO AND CENTRAL AMERICA. 
151 
by moving upward and downward, increase or diminish the size of either pupil; an upward motion 
of the flaps increasing the lower pupil at the expense of the other, and vice versa. (Marsh). 
This division of the pupil into two parts permits the fish, when swimming at the surface of the 
water, as is its usual custom, to see in the air with the upper portion and in the water with the lower. 
It is thus able to see not only such insects as are upon the surface of the water or flying in the air 
above, but also any that may be swimming beneath the surface. 
According to Mr. Nelson the individuals of this species swim always at the surface and in little 
schools arranged in platoons or abreast. They always swim headed upstream against the current, 
and feed upon floating matter which the current brings them. A platoon may be seen in regular 
formation breasting the current, either making slight headway upstream or merely maintaining their 
station, and on the qui vive for any suitable food the current may bring. Now and then one may be 
seen to dart forward, seize a floating food particle, and then resume its place in the platoon. And 
thus they may be observed feeding for long periods. They are almost invariably found in running 
water well out in the stream, or at least where the current is strongest and where floating matter is most 
abundant, for it is upon floating matter that they seem chiefly to depend. They are not known to 
jump out of the water to catch insects flying in the air or resting upon vegetation above the water 
surface, nor do they Seem to feed to any extent upon small crustaceans or other portions of the 
plankton beneath the surface. 
When alarmed — and they are wary and very easily frightened — the)' escape by skipping or jumping 
over the water, 2 or 3 feet at a skip. They rise entirely out of the water, and at a considerable angle, 
the head pointing upward. In descending the tail strikes the water first and, apparently by a sculling 
motion, new impetus is acquired for another leap. This skipping may continue until the school is 
widely scattered. When a school has become scattered, and after the cause of their fright has 
disappeared, the individuals soon rejoin each other. First two will join each other and one by one 
the others will join them until the whole school is together again. Rarely do they attempt to dive or 
get beneath the surface; when they do they have great difficulty in keeping under and soon come to 
the surface again. 
The air-bladder in an example 8 inches long was three-fourths of an inch long and nearly one- 
fourth inch in diameter. It was attached by a slender thread or duct three-fourths inch long to the 
under side of the stomach at the anterior end. The air-bladder lies immediately ventral to the stomach, 
its anterior end lying against and dorsal of the liver, in the tissues of which the duct is immersed. 
No opening from the duct into the stomach could be found in the specimen dissected, though 
more careful dissection of a well-preserved or fresh specimen would probably reveal one. 
The specimens obtained by Mr. Nelson were secured by means of a cast net in the Tehuantepec 
River at Tehuantepec City, which is situated about 50 miles from the sea at an altitude of 200 feet. 
The fish are so wild that it is very difficult to catch them. They jump over a seine readily and will 
not ordinarily take a hook. The cast net seems the most effective means for their capture. 
This fish is viviparous, as is well known. The female (7 inches long) examined by us was collected 
in April, and contains nine young, each about 1.5 inches long. In each of these the yolk sac is quite 
large but flabby, as a result of the absorption of a considerable portion of its contents. In these little 
young the septum in the eye is quite evident and the color pattern of the adult is well shown. After 
the young are born Mr. Nelson observed that they remain for a time in quiet, protected water, not 
venturing into the strong currents until of considerable size. 
Further down the Tehuantepec River near the coast the individuals of this fish are said to be larger 
than at Tehuantepec City. These fish are eaten by the natives, but are not often seen in market, on 
account of the difficulty in their capture. 
22 . Poecilia butleri Jordan. 
Three specimens, 1.37 to 1.67 inches long, obtained by Mr. Deam from a salt-water pool near the 
seashore at Salina Cruz, Gulf of Tehuantepec. 
23. Mollienisia latipinna Le Sueur. 
Nine males and 7 females from Progreso, all excellent specimens, in fine condition. The males 
range in length from 3.75 to 4.63 inches, and the females from 2.25 to 4 inches. Two females from 
Montecristo, 2.25 and 2.75 inches long; and one female from Mujeres Island, 2 inches long. 
24 . Tylosurus notatus (Poey). Needle-fish; Long-jaws. 
One specimen, 13 inches long, from Progreso. Snout broken; measurements not accurate. D. i, 
13; scales 85 before dorsal, about 45 from front of dorsal to caudal. 
