SIREXOTPEI. 321 



Subclass II. DIPNOI. 



Fishes with the skeleton partly cartilaginous, partly 

 osseous; no occipital condyle. Bulbus arteriosus 

 with two longitudinal valves; air-bladder double, 

 lung-like, communicating by a duct and glottis with 

 the haemal side of the oesophagus, with a pulmonary 

 vein, A narrow gill-opening on each side, with a 

 rudimentary gill-cover; some of the branchial arches 

 without gills; gills free, membranaceous. Nostrils 

 double on each side. Intestine with a spiral valve. 

 Optic nerves not decussating. Oviducts distinct. 

 Ventral fins abdominal. 



Cfr. Miiller, Abhandl. Ak.Wiss. Berlin, 1844, p. 201 ; Owen, Anat. 

 Ycrtebr. i. pp. 8, 14. 



One family only- 



SIRENOIDEI. 



Body eel-shaped, covered with cycloid scales, ^'er- 

 tical fins a continuous border to the compressed ta- 

 pering tail. Pectoral and ventral fins subulate. A 

 single maxillary dental plate is opposed to a single 

 mandibular one. Scapular arch attached to the occi- 

 put. Vent not in the median line. No pseudo- 

 branchiae. 



Freshwater fishes of tropical Africa and America. 



Two genera are known : — 



Threo external branehial appendages 1. PnoTorxKRvs, ]>. :V2-2. 



No external biandiial appciulages 'J. Lepidosiukn, p. S'J'2. 



