588 PBOE. T. W. BEIDGE ON THE MESIAL 



JEustTienopteron Foordi, Whiteaves, the radial elements in both 

 the dorsal and anal fins are apparently bisegmental, but the 

 basal segments in each fin are confluent, altbougb three distal 

 segments are distinct and support the numerous fin-rays. In 

 Undina gulo, Egerton, two radial elements are present in each fin, 

 which are fused distally but distinct and divergent proximally ; 

 and in Diplurus longicaudatus, Newberry, the fin-supports of the 

 two dorsal fins have fused into a single piece in each case, which 

 dorsally supports the dermal fin-rays. The fin-supports of 

 Eustheoiopteron Foordi are obviously derived from a primitive 

 bisegmental type ; but it is equally clear in this species, as well 

 as in Diplurus and Undina, that the structures in question have 

 undergone considerable specialization in which concrescence has 

 played an important part. 



In several families of Physostomous Teleosts, viz., the Osteo- 

 glossidse, Muraenidge, Esocidse, Cyprinidse, Salmonidse, and possibly 

 in others, more or fewer of the radial elements of both the dorsal 

 and anal fins are trisegmental; and in this respect, as well as in 

 the relations of the segments of contiguous elements for mutual 

 support, these families more or less closely resemble the Granoid 

 genera Amia and Lepidosteics. Of the five families, the Osteo- 

 glossidae and the Mursenidse are undoubtedly the most primitive 

 in so far as the character of the fin-supports is concerned, and 

 approach most closely to the two Ganoid genera. In the 

 Mursenidse {Conger and Anguilla) all the radial elements are 

 trisegmental; and there is no concentration of fin-rays on the 

 first or last of the series, each element possessing only a single 

 ray. In the Osteoglossidas suppression has slightly modified 

 certain elements to the extent that the last two in the dorsal 

 and anal fins have lost their distal segments. 



In the three remaining families there is a tendency to a 

 variable reduction in the number of radial elements which retain 

 the primitive trisegmental character, the reduction affecting the 

 more anterior and posterior of the series, which in consequence 

 become bisegmental or even unisegmental. The reduction in the 

 case of the anterior elements is undoubtedly associated with the 

 requirements of a firm support for the large and often spinose 

 anterior dermal fin-rays ; in the case of the posterior elements 

 the reduction is clearly due to degeneration, and is invariably 

 associated with the presence of feebly developed rays or their 

 absence (e. g. Barhus). The extent to which reduction modifies 



