354 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.xxiv. 
jaw freely protractile; lips full, the lower either forming a free border 
to the jaw or else attached l\y a frenuni at the symphysis; maxillar}'- 
short, without supplemental bone, slipping for most or all of its length 
under the preorbital; opercular bones entire; branchiostegals, 6 (or 5); 
gill rakers usually slender; gill openings wide, the membranes free 
from the isthmus or ver}- slightly connected; pseudobranchi^e present; 
gills 4, a slit behind the fourth; nostrils round; the openings, 2 on each 
side; dorsal fin single, long, with 8 to 18 usually slender spines, which 
are depressible in a groove; a sheath of scales along the base of the 
anterior part of soft dorsal and posterior of spinous dorsal, this sheath 
separated by a furrow from the scales of the body; anal fin elongate, 
with 3 moderate or small spines and 15 to 35 slender soft raj^s, its form 
and structure differing in the two sexes; ventral fins thoracic, 1, 5; 
pectorals moderate; caudal forked; oviduct opening behind the vent, 
the two apertures always distinctly separated; air bladder large, sim- 
ple; no pjdoric caeca; vertebra3 13 to 19, 19 to 23, 32 to 42. Viviparous. 
The young are hatched within the body, where they remain closel}^ 
packed in a sac-like enlargement of the oviduct analogous to the uterus 
until born. These foetal fishes bear at first little resemblance to the 
parent, being closely compressed and having the vertical fins exceed- 
ingly elevated. At birth the}" are from li to 2i inches in length .and 
similar to the adult in appearance, but more compressed and red in 
color. Since the announcement of their viviparous nature bv Prof. 
Louis Agassiz in 1853 and by Dr. William P. Gibbons in 1854 these 
fishes have been objects of special interest to zoologists. One species 
{IlysterocciTpus traski) inhabits fresh water; one species {Zalemhnis 
rosaceus) descends to considerable depths. These species reach a length 
of from 6 to 18 inches and are very abundant where found. The}'' are 
much used for food, but the flesh is comparatively poor, tasteless, and 
bony. Most of them feed on Crustacea, but one genus (Aheona) is 
partly or wholly herbivorous. The species mostly live in the surf 
along sandy beaches and are confined to California and Japan, their 
origin being evidently Californian. The two Japanese species are of 
separate Californian origin, Dltrema being descended from ancestors 
of Phanerodon and Emhlotoca^ while JVeoditrema must have sprung 
from ancestors of Ilypocritichthys and Ilyperprosoixm. 
a. Embiotocin.e. Spinous dorsal shorter than soft dorsal, of 6 to 11 spines; anal 
spines graduated; marine species. 
h. Scales relatively small, 60 to 70 in lateral line. 
0. Gill rakers numerous, long and slender, about 25 below angle of arch; lower 
lip without frenum; profile of head depressed above eye; mouth oblique, 
short; lower pharyngeals small. 
d. Males with one row of teeth in upper jaw, the lower with few or none; 
females with toothless jaws Neoditrema. 
CO. Gill rakers few, small, about 15 below angle; lower lip with a frenum; mouth 
with teeth in one series; sexes similar; lower pharyngeals rather slender, 
with small teeth Ditreina. 
