XIII PHYLUM CHORDATA 257 



2. Distinctive CharalCteus and Classification. 



The Dipnoi are Pisces ia which the uotocliord is persistent, 

 there are no vertebral centra, and the primary cranium persists 

 with little ossification, but has added to it a number of investing 

 bones. The skull is autostylic, the lower jaw articulating with a 

 palatoquadrate process which is immovably fixed to each side of 

 the skull. There are four to six cartilaginous branchial arches. 

 The dermal fin-rays are slender ossified fibres, and are sup- 

 ported by numerous cartilaginous or ossified pterygiophores. The 

 caudal fin is diphycercal. The paired fins are of the character of 

 " archipterygia." The pectoral arch is a single cartilage with a 

 pair of superficial investing bones on each side. The pelvic arch is 

 well-developed and cartilaginous. There are gills attached to the 

 branchial arches, and in addition a single or double lung opening 

 into the oesophagus by a ventral aperture. The gills are covered 

 over by an operculum. There is a dermal skeleton in the form of 

 overlapping cycloid scales. There is a distinct cloaca. The 

 intestine contains a spiral valve. The auricle and the ventricle 

 are each imperfectly divided into two parts. There is a contractile 

 conus arteriosus, which has a spirally twisted form, and is partly or 

 completely divided internally by a longitudinal septum. The 

 afferent branchial vessels take their origin close together im- 

 mediately in front of the conus. A pulmonary artery is given 

 off from the afferent branchial system on either side ; a pulmonary 

 vein opens into the left division of the auricle. The optic nerves 

 form a chiasma. The oviducts open anteriorly into the coelome. 

 The ova are of moderate size ; segmentation is entire. 



The Dipnoi are classified as follows : — 



Order 1.— Monopneumona. 



Dipnoi in which the lung is single, and the lateral jointeJ rays 

 of the " archipterygium " are well developed.. 



This order comprises only the Australian Ceratodus. 



Order 2. — Dipneumona. 



Dipnoi in which the lung is double, and the lateral rays of the 

 " archipterygium " are vestigial or absent. 



This order includes Protopterus (Fig. 913) of South Africa, and 

 ~ Zepidosiren of South America. 



3. General Remarks. 



The three genera of living Dipnoi are closely allied in all the 

 most essential features of their structure, and it will only be neces- 



