VEBTEBEATA. 11 



a. Skull well developed and with a lower jaw; nostrils paired. 

 b. ]STo median rayed fins ; limbs not developed as rayed 

 fins, but, if present, having the same skeletal elements 

 as in the higher vertebrates; skin usually naked; 

 respiration in the adult chiefly, or entirely, accom- 

 plished by means of lungs, the branchiae rarely 



persistent BATRACHIA. 



bb. Rayed fins present on median line of the body ; limbs 

 developed as rayed fins ; at least one pair being always 

 present ; respiration by means of branchiae throughout 

 life ; lungs not developed. 



c. Membrane bones (operculum, etc.) developed in con- 

 nection with the skull; gills free; the branchial 

 openings a single slit on each side; exoskeleton 

 usually of scales, or plates, never placoid; eggs 



small and numerous PISCES. 



cc. Skull without membrane bones ; gills not free ; the 

 branchial openings slit-like, usually several in 

 number; exoskeleton placoid, never of scales, but 

 usually composed of calcified papillae of various 

 styles ; eggs few and large. . ELASMOBRANCHII. 

 aa. Skull imperfectly developed and with no lower jaw; 

 a single median nostril ; paired fins undeveloped, with 

 no shoulder girdle nor pelvic elements; gills purse- 

 shaped ; skin naked. . . MARSIPOBRANCHII. 

 ft Skull undeveloped, with the notochord persistent and extend- 

 ing to the anterior end of the head ; brain not distinctly 

 differentiated ; heart none, LE?TOCABDII. 



