Phylogeny of tlie TeJeostomi, 337 



changed, but tlie metapterygium became segmented (or tliis 

 segmentation may be primitive, each segment representing an 

 axonost). From such an asymmetrical fin the symmetrical 

 fins of Geratodus would be derived by an increase in length 

 of the lobe and of the number of axial segments and the 

 development of posterior cartilages for the support of the 

 inner series of dermal rays. The evolution of the ventral 

 fins would be on similar lines. 



:)^ >■ 



Fig-. 2. — Diagrams to illustrate the evolution of lobatc paired fins ; the 

 axonosts are unshaded, the baseosts shaded 1, jminitive con- 

 dition ; 2 and 3, sta;ies seen respectively in ventral and pectoral 

 of Acipenser ; 4, obtusely lobate fin ; 5, pectoral of Folypterus ; 

 6, acutely lobate fin. 



In the pectoral tin of Poh/pterus there are two basal 

 pieces articulated to the coraco-scapular ossifications, which 



Traiiuair (Tr. R. Soc. Edin. xxvii. 1876, p. 383, pi. xxxii. fig-. 9), would 

 seem to consist of an axis (nietapteryrriunij of three segments an I of three 

 baseosts, of which the first appears to be attached to the coraco-scapular. 



