APRIL, 1910. FISHES OF CHICAGO MEEK AND HILDEBRAND. 227 



The fishes or fish-like vertebrates of Chicago and its environs belong 

 to two classes: The Marsipobranchii, the lampreys, the hagfishes, 

 and the like; and the Pisces, the sharks, skates, and the true fishes. 

 Of the former but two species occur here; of the latter all are true 

 fishes, there being no sharks or skates. 



The fishes, or fish-like vertebrates, may be defined briefly as cold- 

 blooded animals adapted for life in the water, breathing by means of 

 gills, which are persistent throughout life, and having the limbs, if 

 present, developed as fins, never with fingers and toes. 



The two classes here mentioned may be defined briefly as follows : 



A. Skull imperfectly developed, without true jaws ; gills purse-shaped, 

 not attached to arches ; a single median nostril ; body eel-shaped. 



Marsipobranchii, 231 



AA. Skull well developed, and with true jaws; gills attached to arches; 

 nostrils not median, in one or more pairs. Pisces, 234 



KEY TO THE FAMILIES OF FISHES 



a. Gill openings 7 on each side; mouth circular, disc-like; nostril 

 single, on top of head; no paired fins. Petromyzonida, 231 



aa. Gill openings one on each side; mouth not disc-like; jaws pre- 

 sent ; nostrils in pairs. 



b. Ventral fins, when present, abdominal. 



c. Tail heterocercal, 



d. Mouth under the projecting snout; body naked, or with 5 series 

 of bony plates. 



e. Snout produced into a long oar-like blade ; opercle produced into 

 a long triangular flap; mouth large, not sucker-like. 



Polyodontida , 234 



ee. Snout triangular, not produced into an oar-like blade ; opercle 

 without triangular flap; mouth small, sucker-like. 



Acipenseridtz, 235 



dd. Mouth terminal, jaws about equal; body covered with hard 

 rhombic (ganoid) plates or scales. 



f. Jaws long, beak-like; no gular plate; dorsal rays about 10 ; body 

 with rhombic plates. Lepisosteidce , 237 



ff. Jaws shorter, not beak-like; a gular plate; dorsal rays about 

 50; body with cycloid scales. Amiatida, 238 



cc. Tail homocercal or isocercal. 



