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, RZ, 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- 
3 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 (Ama, p. 163) with herring- 
a _ 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 fin represents the simplest known 
condition of the archipterygium. é/. on p. 248. 
