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 devel- 

 oped 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 Chimseras (Elas- 

 mobranchii), and the lung-fishes (Dipnoi). To the first-men- 

 tioned subclass 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 Mississippi 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. Vertebra? 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. 



f. Mesocoracoid present except in eel-shaped forms; air-bladder with open duct 



when present. 



g. Anterior vertebra 3 not modified, similar to the others, or more elongate, 



separate and not provided with Weberian ossicles. 



