

xiii PHYLUM CHORDATA 257 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Dipnoi are Pisces in which the note-chord 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 diphy cereal. 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 ccelome. 

 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 jointed 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 Jr'rotoplerus (Fig. 913) of South Africa, and 

 Lepidosiren of South Americ^. 



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- 



