586 PBOF. T. W. BRIDGE ON THE MESIAL 



there is not that marked disparity which is so characteristic of 

 Elasmobranchs. Their reduction in number, as well as their 

 increase in size, is presumably due to the fusion of primitive 

 " actinotrichia ;" and, in consequence of the more deeply seated 

 position of the radial elements, they now become the chief support 

 of the external portions of the fins. 



(2) In Amia and Lepidosteus the radial elements exhibit a 

 decided approximation to the condition of these structures in the 

 more generalized Teleosts. They are trisegmental, each element 

 consisting of an ossified dagger-shaped proximal segment, an hour- 

 glass-shaped mesial segment also ossified, and a nodular cartila- 

 ginous distal segment. The various segments afi'ord mutual 

 support to one another, not by their parallelism and apposition, 

 but by the articulation of the mesial and distal segments of one 

 element with the proximal and mesial segments of that next 

 succeeding. A marked reduction in the number of fin-rays has 

 taken place, and each radial element has now but a single ray, 

 which is cleft basally and clips the distal segment of its proper 

 radial element ; but from what has been said as to the articular 

 relations of the segments of contiguous elements, it is obvious 

 that two elements contribute directly or indirectly to the support 

 of each ray. The dermal fin-rays are now the exclusive support 

 of the externally visible portions of the fins, the radial elements 

 having become deeply seated between the dorso-lateral muscles 

 of opposite sides of the body — a position which they retain in the 

 remaining Ganoids and in all Teleosts. Indications of suppres- 

 sion of segments of particular elements are not wanting, and, as 

 in Elasmobranchs, they are characteristic of the more anterior or 

 posterior of the supporting elements of the fins, which, in con- 

 sequence, may become bisegmental or even unisegmental. The 

 fact that in Lepidosteus the first and last of the radial elements 

 of both the dorsal and anal fins support one or two rays, in addition 

 to the single ray which normally belongs to each, is probably 

 due to the concentration of certain rays which have lost their 

 radial elements during the atrophy of a primitively more extensive 

 fin, on the first and last of the persistent residue of the fin- 

 supports. The presence of vestigial radial elements (Amia) 

 between the dorsal and anal fins indicates the primitive continuity 

 of these structures. 



(3) The third type, represented by Polypterus, is of a singularly 

 aberrant character. The simple bisegmental elements of the 



