242 On Suhdwision of the Median Fin in a Dipnoan Fish. 



Such being the arrangement in the Crossopterjgii, it is of 

 much interest to be able to institute comparisons with the 

 Dipnoan fishes, which exliibit so many features of resem- 

 blance to the order just mentioned. Two Devonian genera, 

 Phaneropleuron and Dipterus^ are known to possess a discon- 

 tinuous median fin ; and quite lately specimens of the first of 

 these genera have been found displaying all the fin-supports 

 in undisturbed position. The fossils in question are referable 

 to Phaneropleuron curtum^ were obtained from the Upper 

 Devonian of Canada, and have been acquired by the British 

 Museum. 



It now appears that, at least in the case of the Dipnoan 

 anal fin, the process of separation from the caudal is precisely 

 similar to that observed in the Crossopterygii. As shown by 

 the accompanying illustration 

 the axonosts (a) are fused into a 

 single club-shaped element, and 

 there are three hour-glass- 

 shaped baseosts (5) of nearly 

 equal size. Round the distal 

 ends of these cartilages the 

 clusters of dermal fin-rays (_/) 

 overlap in the usual manner. 

 The same arrangement is also 

 indicated in the figures of Pha- 

 neropleuron curtum already pub- 

 lished by Whiteaves * ; but, 

 owing to the considerable dis- 

 placement of the fin-supports in 

 all the specimens at the dis- 

 posal of that author, the pecu- 

 liarity now described does not 

 appear to have been observed. 



When specimens of Dipterus showing the fin-supports have 

 been discovered the same phenomenon will doubtless be met 

 with also in that genus ; but as yet there is unfortunately no 

 evidence on the subject. 



Skeleton of Anal Fin of Phanero- 

 pleuron curtum, from the Upper 

 Devonian, Scaumenac Bay, P. Q., 

 Canada, a, axonost; b, base- 

 osts; /", bases of fin-rays. [Brit. 

 Mus. no. P. 6785.] 



* J. F. Whiteaves, "Illustrations of the Fossil Fishes of the Devonian 

 Piocks of Canada. — Part I.," Trans. Roy. See. Canada, vol. iv. sect. iv. 

 p. 108, pi. X. fig. 2. 



