184: Bulletin ^7, United States National Museum. 



94. XYRAUCHEN, Eigenmann & KiiRcli. 

 (Razok-back Sucker.) 



Xiiravchen, Eioenmann it Kiuscii, Proo.U.S. Nat. Mus., 1888, 550, ((-//jj/io). 



Characters of the genus Catostomns iu all respects except that behind 

 the occiput is a shai'p-^'^i&cd hump produced by the singularly developed 

 internenral bones. The anterior portion of the hump is supported by a 

 large iuterueural formed by a thick central pillar with anterior and pos- 

 terior wings, the former coming to a point on the medial projecting plate 

 of the Bupraoccipital, forming a large opening beneath it; the latter wing 

 is somewhat smaller and articulates with the second internenral. This 

 interueural is a thin, flat, subrectangular plate ; the next is an irregular, 

 flat plate about I as large as the second, while the next 3 are small, 

 flat plates above and bent forward. Other bones of the anterior part of 

 the skeleton are somewhat modified in form, but the essential character 

 of the genus is iu the great develojimeut of the bones at the nape, show- 

 ing externally as a sharp-edged hump. Two species known ; large suckers 

 of the Colorado basin, {^vpbq, razor* avxvv, nape.) 



a. Scales about 13-73-13 ; dorsal rays 13 or 14. Body deep, the back bigh, the hump com- 

 tneucing at the nape with no scales before it or on its edge. cyi-ho, 300. 



aa. Scales lG-81-13 ; dorsal rays 12 ; body more slender, the liumiJ much lower; scales ppjsent 

 before it and on its edge. i'ncompahore, 301. 



300. XYRAUCHEN CYPHO, (Lockington). 

 (Razor-back Sucker ; IIump-backed Sucker.) 



Head 4 ; depth 4. D. 13 or 14 ; A. 7. Scales 13 to 15-72 to 77-13. Body 

 stout, compressed, the head low, the profile ascending to the prominent 

 hump, which is largest in adult specimens and usually begins at the nape 

 with no scales before it, its anterior edge straight, sharp, and scaleless; 

 mouth wide, inferior ; upper lip with 2 rows of papilL'c ; lower lip deeply 

 divided, with 8 rows. Dorsal long and low, with concave edge; caudal 

 broad and strong, with numerous rudimentary rays; pectorals moderate. 

 Scales loosely imbricated. Pharyngeal teeth small, numerous ; peritoneum 

 black. Coloration plain olivaceous. Basin of the Colorado and Gila 

 rivers ; very abundant where the water is not too cold ; reaching a weight 

 of 8 to 10 pounds, (cypho, hunchback.) 



Oatostomtis cijpho, Lockington, Proc. A<;. Nat. Sci. Phila., 1880, 237, Colorado River at mouth 



of Gila, Arizona. 

 Catoslomns ciipho, Jordan & Gilbert, Synopsis, 129,1883. 

 Xyrauchun cypho, Kirsch, Proc. U. S. Nat. Mus., 1888, 55G; Jordan, Bull. U. S. Fish Com., 1889, 2G. 



301. XYR.iUCHEN UNCOMPAHGRE, Jordan & Evermann. 



Head 4 ; depth 4J. D. 12; A. 7. Scales 16-81-13. Body more elongate 

 than in A', cypho of the same size, the form resembling that of a Gila ; 

 bead flatter, narrower, and less depressed than in X. cypho, the small lips 

 rather larger and more coarsely tuberculate. Nuchal hump much lower 

 than in X. cypho, but forming a sharp keel which does not extend for- 

 ward to the nape, there being 13 scales before it, its surface also scaly. 



