590 PROF. T. W. BRIDGE ON THE MESIAL 



that they result from secondary segmentation of the distal part 

 of the " Flossenstrahltrager." The latter of tlie two suggestions 

 seems the more reasonable ; for the curvature of the " Flossen- 

 straliltrager " is strongly suggestive of the similarly bent shape 

 of an ordinary proximal and mesial segment taken together. It 

 is nevertheless probable that the cutting-ofF of the mesial segment 

 may in some cases precede ossification, while in others it may be 

 the result of the appearance of a separate centre of ossification 

 at the distal end of the " Flossenstrabltrager." Lepidosteus and 

 Amia are, perhaps, examples of the former method, inasmuch 

 as in these genera the cartilage-tipped mesial segments are 

 separated by a very evident suture from the similarly tipped 

 distal extremities of the proximal segments. On the other hand, 

 in Esox (PL XXI. fig. 11), and possibly in other Teleosts with 

 trisegmental elements, the second method has been the one 

 adopted, the mesial segments in the more anterior radial elements 

 of the dorsal fin being represented by small ossific centres in 

 the unsegmented cartilaginous extremity of a backwardly curved 

 " riossenstrahltrager." 



The existence of separable mesial segments in Teleosts, not 

 only in the families above mentioned but also in certain 

 Acanthopterygii, renders it possible to regard the radial elements 

 of Teleosts as typically trisegmental, and therefore directly com- 

 parable with the corresponding structures in Ganoids (excluding 

 Folypterus) and existing Elasmobranchs. 



As regards the relative constancy of the three typical segments 

 of a radial element, it seems reasonable to infer, from the order 

 of their suppression, that not only in the families above men- 

 tioned, but in Teleosts generally, the proximal segment is the 

 most constant, that the distal segment is next constant, while the 

 mesial is apparently the least constant and that most likely to 

 disappear first. 



In the Physostome families the Siluridse, Characinidae, and the 

 Clupeidse the radial elements are either bisegmental or uniseg- 

 mental, never, owing to the absence of a distinct mesial segment, 

 trisegmental : very rarely is it the case, as in some Siluridse (e. g. 

 Cnidoglanis), that a functional dorsal fin has no radial elements but 

 is supported solely by its fin-rays. In the Characinidae ( Githarinus) 

 and the Clupeidse (Cluped) all the radial elements in both fins 

 are bisegmental, consisting of proximal and distal segments. In 

 the Siluridse {Flaty stoma, Amiurus), while the great majority of 



