188 Bulletin 47, United States National Museum. 



being horizontal or nearly so. Lips usually well developed, the form of 

 the lower varying, usually with a slight median fissure, but never deeply 

 incised ; the lips with transverse plicae, the folds rarely so broken up as 

 to form papillae. Jaws without cartilaginous sheath. Muciferous system 

 considerably developed. Opercular bones moderately developed, nearly 

 smooth. Isthmus broad. Gill rakers weak, moderately long. Pharyn- 

 geal bones rather weak, much as in Erimyzon and Catostomus, the teeth 

 rather coarser, strongly compressed, the lower 5 or 6 stronger than the 

 others, which . rapidly diminish in size upward, each with a prominent 

 internal cusp. Scales large, more or less quadrate in form, nearly equal 

 in size over the body, and not specially crowded anywhere, usually 

 about 44 in the lateral line and about 12 series between dorsal and ven- 

 trals. Lateral line well developed, straight or anteriorly curved. Fins 

 well developed, the dorsal inserted about midway of body, its first rays 

 usually rather nearer snout than caudal, the number of developed rays 

 usually about 13, but varying from It to 17. Anal fin short and high, 

 usually emarginate in the male, with 7 developed rays. Caudal fin 

 deeply forked. Air bladder with 3 chambers. Skeleton essentially as in 

 Catostomus, the vertebrae (in M. anisurum) 27 -f- 14 = 41. Sexual peculiar- 

 ities little marked, the males in the spawning season with the lower fins 

 reddened, and the anal rays swollen and somewhat tuberculate. Species 

 /ery numerous in the United States east of the Rocky Mountains. They 

 spawn in spring, running up from the rivers into the smaller streams for 

 that purpose. They are difficult to distinguish and have been unduly 

 multiplied by authors. They are less tenacious of life than the species 

 of Catostomus. (fiv&w, to suck ; aro/ua, mouth ; hence, more properly 

 Myzostoma.) 



a. Lips full, the folds broken up into evident papillae; mouth very small, the snout project- 

 ing. D. 12 to 14. PAPILLOSUM, 304. 

 aa. Lips plicate, the folds not forming distinct papillae. 



b. Dorsal fin large, of 15 to 18 rays; lower lip V-shaped, somewhat papillose. 



c. Mouth moderate ; lower fin pale. ANISURUM, 305. 

 cc. Mouth quite small ; lower fin reddish, COLLAPSUM, 30G 



bb. Dorsal fin smaller, of 10 to 14 rays. 



d. Lower lip narrow, infolded, V-shaped when seen from below, with a distinct 



median crease in which the halves meet, forming an acute angle. 

 e. Body stout, with broad, bluntish head, which is flattened above; dorsal rays 12. 



nucco, 307. 

 ee. Body elongate, subcylindrical, little compressed; snout truncate. 



PIDIKNSE, 308. 



eee. Body compressed, the back elevated; muzzle projecting beyond the very 

 small mouth, COREGONUS, 309. 



dd. Lower lip thin, forming a narrow, crescent-shaped border around the mandible. 

 /. Head small, 5 in length; snout prominent; dorsal rays 12 to 14. ALBUM, 310. 

 ff. Head stout, 4 in length; snout not projecting; dorsal rays 14 or 15. 



THALASSINUBF, 311. 



ddd. Lower lip full, truncate behind, not evidently V-shaped or U -shaped. 



g. Dorsal fin moderate, its rays 12 to 14, usually 13 in number; body more or less 



compressed. 

 h. Caudal fin normal, the 2 lobes similarly colored. 



i. Caudal fin with the upper lobe not conspicuously longer than the 

 lower. 



