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 defined, includes, in addition to the true 

 fishes (Teleostomi) , the sharks, skates, and Chimagras {Elasmohran- 

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

 class belong all American fresh-water fishes and fish-like A^ertebrates 

 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, 



CO. 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. Vertebrge double-concave; maxillary not transversely divided; scales 

 cyjoid ." "... 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. 



