504 EDWIN S. GOODRICn. 



am unable to say. The dermal rays of tlie paired fins are 

 essentially similar to those of the median fins; but, of course, 

 the proximal segment is shorter. 



The dermal rays of Dipterus are extremely interesting. It 

 will be remembered that in this ancient Dipnoan the median 

 fins are of the discontinuous type found in Crossopterygii.^ 

 Ceratodus, Ctenodus, and Urouemus have a single continuous 

 median fin, whilst Phaneropleuron has a separate anal, and 

 Scaumenacia an anal and dorsal. Dipterus has separate 

 short and powerful dorsal, caudal, and anal fins. Now Dollo 

 has argued with great force that the progress of evolution 

 in the Dipnoi has been from the Dipterus type to the Cera- 

 todus type of median fin (9). There are, however, objections 

 to this view, as we shall see later. 



The scales of Dipterus are still cycloid, but they are 

 thicker, and covered with a better developed layer of 

 ganoine than in other Dipnoi. In many specimens the 

 ganoine of the body-scales is not present, having probably 

 been either broken oif or destroyed in the process of 

 fossilisation. 



Pander has studied the dermal skeleton of the fins in 

 detail. According to his account and figures, Dipterus is 

 provided with bony, jointed, and branching rays, which are 

 covered over with small superficial ganoid scales. Moreover 

 he describes the jointed dermal ra^'S as articulated to the 

 extremity of corresponding proximal " Trager " (apparently 

 meaning endo-skeletal fin-supports), which are also covered 

 over with scales similar to those on the body (27). With 

 regard to these proximal " Trager," there can be little doubt 

 that they are merely the bases of the dermal rays. 



After the most careful examination of as many specimens 

 as possible, I have not been able to dispel all doubt from my 

 mind as to the structure of the fins of Dipterus. In the 

 majority of fossils they appear to be as follows : — The fin- 

 membranes are supported by numerous dermal rays, on the 



1 It must not be forgotten, in view of the discubsioii wbicli follows, that 

 Tarassius is said to be a Crossoplerygian with a continuous median fin. 



