THE EVOLUTION OF FINS. 



31 



in direct contact with the " basipterygium," reminding one precisely 

 of the pelvic fins of a modern bony fish. 

 To sum up : — 



(i.) The median fins are primitively supported by a series of 

 rods, in continuation of the series of neural and haemal 

 arches of the axial skeleton of the trunk, 

 (ii.) The supporting rods gradually become atrophied as the 

 dermal fin-rays develop, and are correlated in number 

 with these rays in the higher fishes. 

 b 



Fig. 



Fig. 3. — Anterior a.n^\ fin oi Xenacanthus deckeni. b, c. Ha;mal 

 arches. After A. Fritsch. 



Fig. 4. — Posterior dorsal fin of Holoptychius (Glyptolcpis) Icptop- 

 tents, from the Old Red Sandstone of Nairnshire, showing 

 traces of dermal rays distally. From the Catal. Foss. Fishes, 

 Brit. Mus., pt. ii. 



Fig. 3. 



(iii.) The sub-division of the once-continuous fin results either 



{a) from the loss of portions of the dermal membrane and 



the subsequent degeneration of the supports ; or [b) from 



the crowding together of the supports at two points in the 



dorsal region, and at one or two points in the anal 



region. 



Now, to apply these results to the case of the paired fins. If the 



hypothetical primitive lateral 'fin-fold on each side was originally 



supported by a series of parallel rods of cartilage directed outwards, 



