492 EDWIN S. GOODRIOH. 



It will he seen below, when we come to discuss the sfcruc- 

 tui'e of the extinct forms, that the evidence is overwhelmingly- 

 strong in favour of the theory that the jointed dermal rays 

 or lepidotrichia, of at all events the lower Teleostomes, are in 

 a general sense homologous with the scales of the body. 

 But it will also appear that the problem is really not as 

 simple as it would seem to be at first sight, and that, from 

 what we know of the dermal rays of the Dipnoi, there is 

 some reason to believe that the lepidotrichia may after all be 

 structures of compound origin, formed by the fusion of cera- 

 totrichia with overlying scales (p. 498). 



The dermal rays of the Dipnoi combine certain of the 

 characters of both the ceratotrichia and the lepidotrichia. 

 Whilst they resemble the former in their fibrous and flexible 

 structure, their more or less rounded shape, their deeply 

 inserted proximal ends overlapping the endo-skeleton, and 

 their early development, they are like the lepidotrichia in 

 their possession of bone-cells, of calcification, of joints, and 

 in their situation below the body scales. To decide to which 

 category they belong seems almost impossible from the 

 evidence afforded by the few surviving genera so imperfectly 

 known. The apparent absence of actinotrichia is another 

 difficulty in the solution of this problem, the discussion of 

 which we will defer until the extinct Dipnoi have been dealt 

 with. 



The Dermal Rays op Fosstl Fish. 



The dermal fin-rays of the extinct Elasmobranchs, as has 

 already been mentioned above, differ so little from those of 

 their modern re])resentatives that they need no further 

 description. In Cladoselache, ceratotrichia have been de- 

 scribed and figured by Dean (8) ; but I have not been able to 

 find them in the paired fins. 



Ceratotrichia quite similar to those of the Elasraobranch 

 lish are found in the pectoral fins of AcanLhodians. They 

 liave been described in detail by Reis (31). Kows of small 



