SOUTH AMERICAN NEMATOGNATHI. 11 



anal short. Derm naked. Dorsal and pec- 

 toral spines scarcely developed. 



PygididtE v. 



rt. Dorsal entirely 'in front of the veutrals; vomer with teeth; head com- 

 pressed; anterior nares almost labial; gill membrane broadly united 

 with the isthmus; air bladder capsule entirely free from the skull; occi- 

 l^ital with a prominent ^Drocess. Cetoj^since. 



aa. Dorsal above or behind the ventrals; no teeth on vomer. Head de- 

 pressed. 



b. Gill openings broad, the gill membrane almost free or forming a free 

 membrane across the isthmus; air bladder capsule joined to the skull. 



Pygidince. 

 bb. Gill membrane confluent with the skin of the isthmus, the gill open- 

 ings a narrow slit in front of the pectoral. Stcf/opheUni. 



eee. Air bladder enclosed in a capsule formed 

 by the skull and by the lateral processes of 

 the anterior vertebra?; diaphragm partly or 

 wholly osseous, formed by the expansions of 

 the clavicle and scapular process. Scapula 

 and its process firmly joined to the skull. Gill 

 membranes joined to the isthmus. 

 /. Derm naked, mouth inferior, lower lip re- 

 verted; teeth bicuspid, in several series. 



Argiid^ VI. 

 ff. Derm covered with bony plates. 



g. Caudal vertebraj compressed, the neu- 

 ral and hsemal spines expanded forming 

 a continuous ridge above and below. 

 Sides with several series of plates. Mouth 

 inferior, the lower lip reverted; teeth 

 turned abruptly back above and usually 

 expanded; a single series of teeth erect, 

 the intermaxillaries and dentaries box- 

 shaped, filled with numerous relay teeth; 

 intestinal canal coiled. Cavity of the 

 air bladder communicating with the ex- 



