FISH. 



131 



agrees with the Siluridce in having no coecal appendages, though the air-bladder 

 is similar to that of the ordinary Salmonidce. There are some peculiarities con- 

 nected 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. Aplochiton Zebra. Jen. 



Plate XXIV. Fig. 1. 



A. obscure plumbeus ; fasciis nigris transversis: maxillis cequalibus. 



B. 3; D. 11; A. '2/14; C. 16, &c. ; P. 18; V. 7. 



Long. unc. 9. lin. 6. 



Form.— General form somewhat resembling that of the Mackarel, elongated, and approaching to 

 fusiform. Greatest depth about the middle, equalling one-sixth of the entire length. Line of 

 the back and profile nearly straight, the latter falling very little. Greatest thickness rather 

 more than half the depth. Head small, contained about five and a half times in the entire 

 length. Snout short, but rather acute. Mouth with a moderate gape reaching to beneath the 

 anterior angle of the eye : when shut, both jaws equal, the lower one ascending at an angle of 

 45° to meet the upper ; 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 jaws: a double row of similar teeth, 

 but stronger and more curved, down the middle of the tongue ; also a double row along 

 the middle of the vomer. Eyes moderate ; their diameter four and a half times in the length 

 of the head ; distant about one diameter and a quarter from the end of the snout, 

 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 of 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 the pieces of the opercle present, but the interopercle only just appearing 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 cut straight and vertical. 



The whole skin perfectly naked everywhere, without the least vestige of scales. No lateral 

 line, except a faint streak, passing along the middle of the sides, be so called. Dorsal com- 

 mencing at the middle of the length, this last being measured to the base of the caudal fork ; 

 of the same form as in the ordinary species of the genus Salmo ; its greatest height a little 

 exceeding its length, which last is rather more than half the depth of the body ; first ray simple, 

 the rest branched. Adipose small, and just halfway 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 terminating opposite 



