I;/
Si '
agrees with the S ilu rid a im having no ccecal appendages, though the air-bladder
is similar to that of the ordinary Salm onida. There are some peculiarities connected
with the anal and sexual orifices which I shall notice presently, along with
other points in the internal structure, in the species to be described first.
Mr. Darwin has brought home two species of this new genus, agreeing precisely
in form, but very distinct in respect to size and colouring.
1, A p l o c h i t o n Z e b r a . Jen .
P l a t e XX IV . F ig . 1.
A . ohscurh plum teus ; fa sciis nigris transversis: m axillis mqualilms.
B. 3 ; D. 11 ; A. 2/14; C. 16, & c.; P . 18; V. 7.
L o n g . u n c . 9 . lin . 6 .
F o r m — General form somewhat resembling that of the Maclmrel, elongated, and approaching to
fusiform G reatest depth about the middle, equalling one-sixth of the entire length. L ine of
the baek and profile nearly straight, the latter falUng very little. Greatest thickness rather
more than half the depth. H ead small, contained about five and a half times m the entire
len ith . Snout short, but ratber acute. M outh with a moderate gape reaching to beneath the
anterior angle of the eve; when shut, both jaws equal, the lower one ascending at an angle of
45" to meet the u pp er; when open, the lower one a little the longest. Margin of the upper
jaw formed by the intermaxillary, the maxillary appearing behind it. A single row of small
but sharp teeth extending along the entire margins of both jaw s; a double row of similar tooth,
b u t stronger and more curved, down the middle o f the tongue; also a double row along
the middle of the vomer. Eyes moderate ; their diameter four and a half times m the length
of tho head ; distant about one diameter and a quarter from^ the^ end of the snont,
and with an interval between them of about one and a half. Nostrils with two orifices, both
roundish, one before the other, with a little interval between, the posterior one rather the
largest. Two distinct pores on each side o f the crown, one behind the other with an interval
between, above and rather behind the eyes. Gill-opening very large, the membrane with only
three flattened rays, deeply notched beneath, the notch reaching to beneath the middle of the
eyes All Üic pieces of the opercle present, but tho interoperele o n ly ju st appeai'ing behind
the angle of the preopercle. and the subopercle forming but a narrow lanceolate lamina beneath
the true opercle, which last constitutes the greater portion of the gill-flap, and is of an oblong
form, the posterior margin being out straight and vertical.
T he whole skin perfectly naked everywhere, without tho least vestige of seales. N o lateral
line, e.xccpt a faint streak, passing along the middle of tho sides, be so called. Dorsal commencing
at the middle of the length, this last being measured to tbe base of tbe caudal fo rk ;
of tho same form as in the ordinary species of tbe genus Salmo; its greatest height a little
exceeding its length, which last is rather more than half tho depth of the b ody; first ray simple,
the rest branched. Adipose small, and just halfw ay between the end of the dorsal and the
base of the caudal. Anal of a somewhat triangular form, the margin sloping very much off
backwards, commencing a little beyond the tip of the reclined dorsal, and lerm inating opposite