518 PISCES [CH. xx 



Conus distinct in heart with a spiral longitudinal valve, atrium 

 divided into right and left auricles, an inferior vena cava present 

 and pulmonary veins returning blood from air-bladder to heart. 

 An optic chiasma in brain. Oviduct opens by a funnel into coelom. 

 A spiral valve on intestine, air-bladder opens in mid-ventral line of 

 pharynx. 



Family 1. Monopneumona. Air-bladder undivided, young 

 without external gills. 



Ex. Ceratodus. 



Family 2. Dipneumona. Air-bladder bilobed, young with 

 external gills. 



Ex. Protopterus, Lepidosiren. 



