HYBOGNATHUS 115 



falling far short of orbit, scarcely exceeding as a rule the vertical from an- 

 terior nostril-opening; lower jaw with a hard sharp edge and a noticeable 

 protuberance just inside the mouth at the symphysis of the mandibles; jaws 

 about equal; isthmus less than pupil. Teeth 4-4, narrow, with little grinding 

 surface and very little hook; intestine extremely variable in length, from 

 3.7 to 8 times length of head and body, being as a rule over 5; peritoneum 

 dusky. Dorsal fin with usually 8 rays, occasionally 7, set slightly in front 

 of ventrals, usually a little nearer muzzle than base of caudal; longest dorsal 

 ray 1 to 1.2 in head; anal rays 8 (rarely 7); pectorals 1.1 to 1.4 in head; ven- 

 trals falling far short of vent in adults. Scales 5, 37-39, 4, large and rounded, 

 the exposed surfaces little deeper than long; lateral line complete, and nearly 

 straight except for a slight downward curve in front of ventrals; scales before 

 dorsal 13 to 16. 



This species is generally distributed throughout the state, 

 occurring in all our stream systems, including those of the Michi- 

 gan drainage, but most abundantly in those of southern Illinois. 

 It is essentially a river species — one of the few Illinois minnows 

 occurring in larger ratio in rivers than in creeks. It is most 

 abundant in rivers of the second class (coefficient, 2.18), and 

 next in creeks (1.91), but we have also found it not verj^ in- 

 frequent in the lakes and ponds of the river bottoms (.43). 



In general distribution it ranges from Delaware to Georgia 

 and Alabama, and from thence southwest to the Rio Grande, 

 north to the tributaries of the Missouri in the Dakotas, and to 

 the Red River of the North. In this state it is often found in 

 deep and muddy waters, and less frequently than most minnows 

 in swift and gravelly streams. It is one of the five Illinois 

 species found most generally over a mud bottom, its frequency 

 coefficient being 1.68. It has the long intestine, the simple 

 pharyngeal teeth with a well-developed grinding surface, and 

 the few and short gill-rakers characteristic of the mud-eating 

 minnows, and its food corresponds to these structural peculiar- 

 ities. According to our observations the intestine is always 

 filled with fine mud, containing only filamentous algae, diatoms, 

 and other vegetable forms likely to be found on a mud bottom. 

 It is frequently seen in large schools of from fifty to one hundred 

 in deep and quiet water, always lying nearer the bottom than 

 the top, or moving slowly along the bottom as it feeds. The 

 chisel-shaped lower jaw tipped with cartilage is probably used 

 for scraping up the mud and ooze. 



The sexual differences of this species are not striking, al- 

 though the spring males have the nuchal region somewhat 

 swollen, and the top and sides of the head beset with very minute 

 tubercles. Females greatly distended with eggs have been 



