GANOIDS AND TELE OS TS 



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 valved 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 and 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\ef. on p. 248. 

 I, 



