CLASS PISCES 13 



Class PISCES 



(fishes) 



Skull well developed, separate from the vertebral column; a lower jaw, 

 or both upper and lower jaws, developed; limbs typically present and 

 developed as fins, in rare cases (Apodes, etc.) wanting through atrophy; 

 shoulder girdle usually present, rarely obsolete; pelvic bones present 

 (as a rule absent, or represented by rudiment or vestige. — Bridge, Cam. 

 Nat. Hist., p. 475); gills attached to bony or cartilaginous gill-arches; 

 nostrils paired. 



The class Pisces as here denned, includes:, in addition to the true 

 fishes (Teleostomi), the sharks, skates, and 'Chimasras (Elasmobran- 

 chii), and the lung-fishes {Dipnoi). To the first -mentioned sub- 

 class belong all American fresh-water fishes and fish-like vertebrates 

 above the lampreys. The relation borne to each other by the 10 

 orders of Teleostomi represented in the waters of the central Missis- 

 sippi Valley may be expressed in the following analytical key. 



Key to Orders of TELEOSTOMI 



(The definitions following will in some cases not apply to species not occurring 

 in Illinois.) 



a. Tail strongly heterocercal throughout life; some fins usually with fulcra; 



arterial bulb muscular and with numerous valves (not less than 3) ; optic 

 nerves forming a solid chiasma; air-bladder with a well-developed duct. 



b. Skeleton cartilaginous; ventrals with an entire series of basilar segments. 



c. Maxillary and interopercle obsolete; skin naked; air-bladder cellular. 



Selachostomi. 



cc. Maxillary and interopercle present; skin with 5 series of bony shields; air- 

 bladder simple Chondrostei. 



bb. Skeleton bony; ventrals with basilar segments rudimentary; air-bladder 

 cellular. 



d. Vertebrae concavo-convex; maxillary transversely divided into several 



pieces; scales rhombic enameled plates Rhomboganoidea. 



dd. Vertebrae double-concave; maxillary not transversely divided; scales 

 cyloid '. Cycloganoidea. 



aa. Tail homocercal, diphycercal; arterial bulb thin, with a pair of opposite 

 valves; optic nerves crossing, not forming a solid chiasma; duct to air- 

 bladder slender or obsolete. 



e. Ventral fins abdominal, if present, (the pelvic girdle being present and ab- 



dominal in forms which lack ventrals) ; mostly soft-rayed forms. 



