332 
U. S. P. R. R. EXP. AND SURVEYS—ZOOLOGY—GENERAL REPORT. 
HYODON, Lesu. 
Gen. Char. —Body very much compressed, deep upon its middle, covered with large scales, deeper than long, disposed upon 
transeverse oblique series ; ventral line sharp, but not serrated. Head small; snout rounded; jaws equal. Teeth on the jaws 
vomer, palatines, and tongue. Eyes very large. Gill apertures continuous under the throat. Dorsal fin opposite the anal 
Caudal fin furcated, else crescent-shaped posteriorly. Lateral line nearly straight, running along the middle of the flanks. 
Syn. — Uijodon , Lesu. in Journ. Acad. Nat. Sc. Pliilad. I, i, 1818,364. —Rafin. Iehth. Ohiens. 1820,41. — Cuv. Regn. Anim. 
2d ed. II, 1829; &, ed. illustr. Poiss. 283.— DeKay, New Y. Faun. IV, 1842,265.— Storer, Synops. 184C, 
210.—Cuv. & Vae. Hist. nat. Poiss. XIX, 1846, 307. 
Glossodon, Heck, in Eusseg. Reisen, I, h, 1842, 1033. 
Rafinesque has a genus Glossodon, which Heckel appears to have ignored ; adopting Lesueur’s 
genus Hyodon, he sub-divides it into three subgenera: Amphiodon, Glossodon, and Clodalus. 
It is notour present purpose to inquire into this subject beyond introducing here, into its natural 
family, a fish that was supposed at one time to belong to that of Cyprinoids, with the 
peculiar and exceptional characters of having teeth upon the tongue. 
HYODON TERGISUS, Lesu. 
Moon-eye. 
Plate LNXY, Pigs. 1—4. (By error on the plate : figs. 4—7.) 
Spec. Char —Head contained five times and a half in the total length ; snout rounded, sub-conical. Posterior extremity of 
maxillar bone extending to a vertical line drawn posteriorly to the pupil. Eye very large, sub-circular; its diameter entering 
about four times in the length of the side of the head. Anterior margin of dorsal fin somewhat nearer the tip of the caudal than 
the occiput. Base of anal fin entering about four times in the total length. Insertion of ventrals nearer the extremity of the 
snout than the terminus of the anal. 
Syn. — Ilycdcn tergi&us, Lesu. in Journ. Acad. Nat. Sc. Philad. I, i, 1818, 366.— Richards. Faun. Bor. Amer. Ill, 1836,235.— 
Kirtl. Rep. Zool. Ohio, 1838,170, and 195 ; &, in Bost. Journ. Nat. Hist. V, in, 1846, 338.— DeKay, New Y. 
Faun. IV, 1842, 265; pi. xli, fig. 130.— Storer, Synops. 1846, 210.—Cuv. & Val. Hist. nat. Poiss. XIX, 
1846, 309. 
The “moon-eye,” or “lake” and “river herring,” as this fish is sometimes called, has been 
made the subject of much comment by the various writers who have spoken of it. Several 
species have been put on record without criticism ; adopted by some, rejected by others, so that 
up to the present day we are still left in doubt as to the number of species of the genus Hyodon 
inhabiting the fresh waters of North America. Let us hope that the time is not far when the 
Museum of the Smithsonian Institute shall he in possession of preserved specimens from every 
locality where these fishes are found, so as to enable us to institute a thorough examina¬ 
tion of their zoological as well as anatomical characters, in order to settle a question so much 
controverted. 
One fact is already clear to our mind, that the differences in the outline of the anal fin 
alluded to by Lesueur and Valenciennes are traits indicative of the sexes, and should H. clodalus 
prove specifically distinct from H. tergisus the distinction will be based upon very different 
characters. The above diagnosis, which we offer, of H. tergisus, is drawn up with a view of 
assisting future observers should they be prevented from forwarding specimens to Washington 
for ulterior comparisons. 
