GANOIDS AND TELEOSTS 



145 



those of Dipterus. The shoulder girdle includes outer 

 dermal elements, DSG. The external parts of the unpaired 

 fins are dermal ; but their cartilaginous supports are re- 

 tained, RB, even in the tail region. The caudal fin may- 

 be regarded as either diphycercal or heterocercal. The 

 exposed parts of the paired fins, it is especially interesting 

 to note, are only in part dermal ; the two rows of carti- 

 laginous supports are retained in a condition very similar 

 to that of sharks, R B;* two of the basal elements of the 

 pectoral fin, however, have retained the rod-like form in 

 strengthening the front and hinder margin of the fin. 



In visceral structures the Ganoids exhibit the fol- 

 lowing noteworthy characters : a greater number of gill 

 arches ; a spiracle ; a short and almost straight digestive 

 tube, with spiral vaived intestine ; a shark-like pancreas ; 

 an arterial cone, with many rows of valves ; a cellular air- 

 bladder, like that of a Dipnoan ; primitive conditions in the 

 urinogenital apparatus ; shark-like characters in the ner- 

 vous system and sense organs ; a chiasma of the optic 

 nerves, (pp. 260-279). 



Relationships mid Descent 



Johannes Miiller, when separating Ganoids from Tele- 

 osts, recognized clearly even at that early date (1844) that 

 the majority of the structural differences of these forms 

 were bridged over in exceptional instances ; there were 

 thus Teleosts with bony body plates, as well as, it was 

 afterwards found, a Ganoid {Ainia, p. 163) with herring- 

 like cycloidal scales. But he believed that three structural 

 characters of the Ganoids separated them constantly from 

 all Teleosts, and warranted the integrity of the groups. 



* Contrast Gegenbaur's view that this tin represents the simplest known 

 condition of the archipterygium. J\e/. on p. 248. 

 I. 



