68 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



as wide. The intestines of both species vary much in length. Those 

 of IT. nuchalis are usually from 7 to 10 times as long as the whole body, 

 but are occasionally shorter. 



H. argyritis has a large eye, its diameter exceeding the length of the 

 snout. The mouth is small, but more oblique than in the other species; 

 while the lower jaw is fully equal to the upper. The intestines are 

 shorter than in H. nuchalis, being generally about 4£ or 5 times as long- 

 as the body. Now and then one is found with the elementary canal 1\ 

 times the body. The most obvious distinctions between the two species 

 are to be found in the size of the eye and the position of the mouth, 

 whether terminal or inferior. 



The specimens of Hybognathus that I collected at Macon, Miss., and 

 described as above cited, belong here. Those collected at Enterprise 

 were properly assigned to the next species. 



Numerous specimens collected at Memphis, Vicksburg, Edwards, 

 Jackson, Vaughan's, and Grenada. 



36. Hybognathus argyritis Grd. — Silvery Minnow. 



These appear to be quite as abundant as the preceding. Found at 

 the same places. 



Tirodon, gen. no v. 



Pharyngeal teeth 2, 4-4, 2; compressed, not hooked, and with a broad 

 triturating surface. Pharyngeal bones broad and sharply curved. In- 

 testinal canal elongate and convoluted. No barbels at the angles of 

 the mouth. Upper jaw protractile; both jaws tbin. Dorsal over the 

 ventrals. 



(Etymology: zstpw, to wear away; ddoyc, tooth.) 



This genus is undoubtedly very closely related to Hybognathus, differ- 

 ing apparently in no important respect except in having two rows of 

 pharyngeal teeth instead of one. This character has hitherto been 

 regarded as of sufficient value to distinguish genera, and I follow cus- 

 tom. The intestinal canal in the specimen in my possession is not so 

 elongate as in species of Hybognathus, being but 3i times the length of 

 the body; but this canal in the latter genus varies so greatly in length 

 in individuals of the same species that its shortness in the case before 

 us may be an individual peculiarity. 



37. Tirodon amnigenus, sp. no v. 



Form and general appearance much as in Hybognathus argyritis. In 

 the single specimen known the head is broad while the body is much 

 compressed. This may be due to mechanical injury to the specimen. 

 Head long, contained in the body 3£ times. Both upper and lower jaws 

 thin, the lower slightly the longer. Mouth rather more oblique than in 

 Hybognathus argyritis, rather small, the maxillary not extending back 

 to a vertical from the front of the eye. Eye contained in the head 3 

 times, equal to the snout. 



