THE DEHMAL FIN-KAYS OF FISHKS. 511 



the Dipnoi, and those Dipnoi which most closely resemble 

 the Teleostomi, are most likely to possess the most primitive 

 type of dermal rays. Now Phaneropleuron or Scaumenacia 

 on the one hand, and Glyptolepis or Holoptychius on the 

 other, fulfil these conditions. It has been shown above that 

 their dermal rays are of very similar character (p. 501). They 

 are very numerous, closely packed, slendei^, elongated rods, 

 tapering gradually towards the periphery, and branching 

 occasionally, but not very much. They are provided with a 

 long proximal unsegmented piece, deeply embedded in the 

 body, overlapping the endo-skeletal fin-supports, and covered 

 over by the body-scales. The bony nature of these rays may 

 be considered as an expression of the general tendency, so to 

 speak, of these fish towards the ossification of all their 

 skeletal structures. The breaking up into joints of that 

 portion of the ray which projects beyond the body might 

 follow as a necessary consequence of the hardening through 

 ossification. 



According to this view, then, the ceratotrichia and the 

 basal plates of the denticles of Elasmobranchs would be 

 represented in the Dipnoi by the camptotrichia and the 

 superficial scales. 



But in the Teleostomi we nnist suppose that these two 

 originally separate elements have become so closely connected 

 that they have finally fused with each other, giving rise, in 

 the web of the fin, to jointed dermal rays with scale-like 

 upper surface. Each segment of the lepidotrich would thus 

 have been formed b}^ the combination of an elongated ganoid 

 scale Avith a segment of the underlying bony ray. The 

 proximal unjointed region of the ray, embedded in the body, 

 and separated from the body-scales by a considerable thick- 

 ness of soft tissues, would never have fused with the scales, 

 and thus would acquire neither the joints nor the ganoine 

 covering. The structure of Glyptolepis, perhaps one of the 

 most primitive of the known Crossopterygians, and of Cheiro- 

 lepis, the most ancient of Actinopterygians, would certainly 

 agree most remarkably with this interpretation. 



