128 RELATIONSHIPS OF LUNG-FISHES 



have now been closened by the proof that their paddle- 

 shaped fins may be directly deduced from a " monoserial 

 archipterygium," and that their diphycercal caudal, formerly 

 regarded as most primitive in plan, may have been acquired 

 secondarily after a condition of heterocercy (W. N. Parker, 

 Traquair, Dean). 



The resemblances of Dipnoans to Elasmobranchs might 

 be summarized in the following structures : - 



I. VERTEBRAL AXIS. Its notochordal condition and 

 simple metameral, neural, and haemal elements suggest the 

 conditions of Cladoselache (p. 80) ; in that ancient form, 

 however, the vertebral processes had not come into rela- 

 tion with the unpaired fins. 



II. SKULL. The chondrocranium is as yet largely re- 

 tained ; as yet no dentigerous membrane bones of the 

 mouth rim (maxillary and premaxillary) have appeared. 



III. TEETH. These are clearly of an elasmobranchian 

 order; the tubercles of the dental plates (Fig. 125) suggest 

 closely a shagreen pattern ; in Phaneropleuron, marginal 

 cusps have even been retained. The palatine and splenial 

 plates parallel strikingly some of the forms of Cestraciont 

 dentition. 



IV. BRAIN. Its structures are of an advancing elasmo- 

 branchian order, annectent with reptilian (Ceratodus) and 

 amphibian types (Protopterus).* 



V. VISCERAL CHARACTERS. Heart, gills, digestive tract, 

 vessels, mesenteries. 



The closely corresponding characters of Phaneropleuron 

 and Pleuracanthus might be looked upon as independently 

 acquired ; bnt in view of the many nearnesses of their 

 phyla, these characters may reasonably be regarded as 

 proof of genetic kinship. 



* Burckhardt. 



