FISHES OF EL SALVADOR 
263 
seen in small schools. They are rather shy, and when frightened they sometimes 
make leaps of a meter or so above the water, or, more usually, they lift the head 
and body above the surface, leaving only the tail in the water. In this way they 
make rapid headway, and repeated efforts at surrounding a school with a 9-meter 
seine failed. 
The four-eye apparently is never used for food, although it reaches a much 
larger size than several other species that are eaten. The largest fish seen by the 
collectors was 235 millimeters long, but according to the natives at Lake Guija 
the species attains a length of at least 300 millimeters. The male appears to be 
smaller than the female, as no males occurred among the larger individuals examined. 
The largest male seen was 180 millimeters long. 
The species is viviparous, and the anal fin in the male serves as a copulatory 
organ. As the fish develops and reaches sexual maturity the anterior rays of the 
fin become enveloped in scaly skin, which extends to the end of the longest rays, 
forming a sort of tube, and having an orifice distally. A thin membranous recep- 
tacle lies in the abdominal cavity at the base of the anal, which is entered by the 
posterior end of the testes. A tube extends into the modified anal fin from this 
receptacle. This tube lies either right or left of the eveloped fin rays, through 
which the seminal fluid is conveyed to the distal orifice. The ovary is single, and 
the number of young produced at one time appears to be rather small. One ovary 
contained 6 embryos, each approximately 11 millimeters long; another inclosed 
only two embryos, each about 14 millimeters long. 
This fish appears to feed on algae and small entomostracans, insects, and other 
animals found among these plants at the surface of the water. The contents of 
the stomachs examined and the habit of surface feeding and swimming indicate 
that this species may be of value as an eradicator of mosquito lai'vag. 
This fish is known from Southern Mexico, Guatemala, and El Salvador. The 
type locality given by Gill, who first described the species, is the "Pacific coast of 
Panama," where the species quite certainly does not occur. It is more probable 
that the single specimen upon which the description was based came from Guate- 
mala or Mexico, where Captain Dow, for whom the species is named, also collected. 
The specimens at hand were collected in Lake Guija and its outlet, Rio Lempa at 
Suchitoto and San Marcos, Rio San Miguel at San Miguel, Lake Olomega, and 
Cutuco. It was also seen in the Rio Sucio at Sitio del Nino, but no specimens 
were obtained there. 
Order PERCOMORPHI 
Family VII. ATHERINID^E 
The Silversides 
Body elongate, more or less compressed; premaxillaries protractile; jaws 
with two or more series of conical teeth; lateral line usually absent, never complete; 
two well-separated dorsal fins, the first formed of 3 to 6 slender spines, the second 
with a short spine and 7 to 13 branched rays; caudal fin forked; anal fin with a 
single spine and 12 or more branched rays; ventral fins abdominal, each with a 
