VERTEBRATA. 11 



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

 h. No 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 Batraciiia. 



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

 developed as rayed fins; at least one pair being always 

 pi-esent ; respiration l)y means of brauchiaB 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. . Elasmobkanchii. 

 «fl^. 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. 

 XX Skull undeveloped, with tlie notochord persistent and extend- 

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

 ditferentiated ; heart nooe. Leptocardii. 



