FISHES SALMONIDAE THALEICHTHYS STEVENSI. 325 



THALEICHTHYS, Girard. 



GEN. CHAR. Body elongated, sub-fusiform in profile, terminated by a furcated caudal, and covered with scales of moderate 

 development. Anal fin opposite the adipose. Ventrals inserted in advance of the anterior margin of the dorsal. Head elon 

 gated, tapering towards the snout. Mouth very deeply cleft ; jaws toothless ; a small group of minute veivet-Jike teeth upon 

 the front of the vomer, and one series along either pterygoidian bone. Teeth may also be observed at the base of the tongue. 

 Branchial apertures continuous under the throat ; branchiostegal rays eight on either side. 



This genus is instituted for a species whicli has the external appearance of an Osmerus, 

 except in the insertion of the ventrals, which is placed more in advance of the dorsal fin than 

 usual in the latter genus. The mouth is also more deeply cleft, and the jaws are toothless, as 

 stated to be the case in Argentina, 1 although maxillar teeth are represented on Plate 624 of the 

 work just referred to in the case of Argentina leioglossa. 



From Argentina it differs by the presence of teeth on the pterygoidian bones, whilst the pala 

 tines are toothless ; in Argentina the pterygoidians being toothless and the palatines provided 

 with one series of small teeth. 



Thus the present genus stands as a connecting link between Osmerus and Argentina; and 

 should the characters upon which it is based not be found adequate to entitle it to a place in 

 the ichthyic method, then Argentina and Osmerus could no longer be held as differing from one 

 another. 



THALEICHTHYS STEVENSI, G- r d . 

 PLATE LXXV, FIGS. 1 4. (Under the name of Osmerus stevensi.) 



SPEC. CHAR Head sub-conical and pointed. Mouth large ; posterior extremity of maxillar bone extending to a vertical line 

 drawn posteriorly to the orbit. Eye rather small. Adipose fin placed opposite the posterior portion of the anal, which is 

 quite elongated. The insertion of the ventral fins is situated considerably in advance of the anterior margin of the dorsal. 

 Scales moderate, sub-elliptical. Dorsal region dark greyish olive ; middle of flanks yellowUh orange dotted with black ; belly 

 yellowish, unicolor ; upper surface and sides of head greyish ; fins unicolor. 



The head constitutes about the fifth of the total length, in which the caudal enters six times. 

 The snout is sub-conical and slender, the lower jaw protruding beyond the upper, the gape of 

 the mouth being oblique, and the posterior extremity of the maxillar bone, which is rounded, 

 extending to a vertical line drawn beyond the entire orbit. The posterior edge of the gill covers 

 is rounded off, the opercle and sub-opercle being very largely developed. The branchial aper 

 tures are very wide, extending forwards as far as a vertical line drawn through the pupil. We 

 count eight branchiostegals, as many as in certain species of Osmerus. 



The body is quite slender and elongated, compressed and tapering gradually towards the 

 caudal fin, which is shorter than the head. The dorsal fin is much higher than long ; its ante 

 rior margin is somewhat nearer the insertion of the caudal than the extremity of the snout. 

 The adipose is placed opposite the posterior portion of the anal fin, although the Lips of the rays 

 of the anal extend somewhat further back, in which respect the figure on plate LXXV is not 

 quite correct. The anal fin itself is much longer than deep, its base entering five times and 

 three quarters in the entire length. The ventrals are broad and well developed, posteriorly 

 rounded off, inserted considerably in advance of the anterior margin of the dorsal ; hence nearer 

 the extremity of the snout than the base of the caudal, while their posterior extremities do not 



1 Histoire naturelle des Poissons, XXI, 1848, 409. 



