Thylogeny of the Teleostomi. 



137 



changed, but the metapterygium became segmented (or this 

 segmentation may be primitive, each segment representing an 

 axonost). From such an asymmetrical tin the symmetrical 

 fins of Ceratodus 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 

 tins would be on similar lines. 



Minimi/ 



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

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

 dition; 2 and 3, stages seen respectively in ventral and pectoral 

 of Acipenser ; 4, obtusely lobate tin; 5, pectoral of Polypterus ; 

 (j, acutely lobate fin. 



In the pectoral tin of Polypterus there are two basal 

 pieces articulated to the coraco-scapular ossifications, which 



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

 seem to consist of an axis (metapterygium) of three segments an I of three 

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



