FISHES OF COLORADO 43 



Hybognathus nuchalis Agassiz 

 Silvery Minnow 



Hybognathus nuchalis Agassiz, Amer. Journ. Sci. Arts, p. 224, 1855 (Quincy, Illinois); Juday, 

 Bull. U.S. Fish Com. for igo4, p. 226, 1905 (Boulder; Longmont); Judav, Univ. Colo. Studies, 

 Vol. II, p. 113, igo3 (Boulder; Longmont). 



Hybognathus nuchalis placita (Girard) — Jordan, Bull. U.S. Fish Com., Vol. IX, pp. 8 and 17, 

 1889 (Denver; Pueblo). 



Body elongate and compressed; head subconic, rather long, its length equal- 

 ling or exceeding the greatest depth of the body; depth 4 to 4. 5, or 5 (young), 

 head 3.8 to 4. 75 in the length to the base of the caudal; eye medium, 3.8 to almost 

 S in the head (Table III); snout rather prominent, tip of the snout rounded, pro- 

 jecting slightly beyond the upper jaw; mouth small, terminal, slightly ventral, 

 lips thin, angle of the mouth not reaching the level of the anterior margin of the 

 eye by half the diameter of the eye or more; inside of the median portion of the 

 lower jaw enlarged at the tip, into a hard, more or less conical protuberance, so 

 that the jaw when seen from in front is A-shaped; lower jaw shorter than the 

 upper and obtuse at the tip; dorsal fin short and high, the length of its base almost 

 2 in the length of its longest ray; base of the first ray of the dorsal distinctly 

 in front of the level of the ventrals; dorsal of 8, sometimes 7 rays; pectorals short, 

 about 1.25 in the head, not reaching the ventrals by almost the length of the 

 latter; ventrals just reaching the anal opening; anal fin short, of 8, rarely 7 rays; 

 caudal peduncle tapering, its least depth about 2 in the head, caudal fin moder- 

 ately broad and distinctly forked; scales moderately large and well imbricated, 

 5 or 6, 36-40, 4; lateral line complete and prominent, straight, little if at all 

 decurved in the pectoral region, pores large; size rather small, length up to 6 

 inches, average adults 3 or 4 inches in length. 



Color above the lateral line pale olivaceous to brownish green, with a distinct 

 dusky mid-dorsal stripe, sides and body below the lateral line silvery with a steel- 

 blue iridescence; ventral parts cream color to silvery; scales in the region just 

 above the lateral line with numerous fine blue-black chromatophores overlying 

 an indistinct dusky stripe (this stripe is quite distinct or even prominent in pre- 

 served specimens, but usually obscured in living specimens by the silvery color 

 of the sides) ; fins hyaline, rays outlined with dusky. 



A western subspecies of this Minnow, known as Hybognathus nuchalis placita 

 (Girard), has been recognized by some writers on the basis of the smaller eye, its 

 diameter in this variety being 5 in the head. Such specimens were found in the 

 collections examined but, associated as they were with specimens having larger 

 eyes and not being confined to a particular drainage area, the subspecies is not 

 separated from the species proper in this report. Table III gives the number of 

 times the eye was contained in the head for several series of specimens. 



Since the specimens with the small eyes occur in the collections from the 

 Arkansas River drainage with one exception and the specimens from the Platte 



