298 J. K. Thacher — Median and Paired Fins. 



The testimony of the Ganoids and Teleosts seems to me to be 

 somewhat adverse to the theory. Again it is impossible to think 

 that that of the Stapedifera can be very clearly in its favor, when 

 Huxley, while accepting the archipterygium as the parent form, 

 gives an explanation of the cliiropterygium entirely distinct from and 

 utterly inconsistent with that of Gegenbaur. 



Any opinion adverse to the archipterygium theory will have diffi- 

 culty in maintaining itself, so long as it does not show that the 

 resemblance between the fins of sharks, and those of Dipnoi is a 

 merely superficial one, and is not able to suggest how a certain show 

 of resemblance might have arisen in two entirely distinct and different 

 series of developments. 



Another View of the Origin of Vertebrate Limbs. 



Into competition with this theory, which sees in the fin of Cera- 

 todus that from which all other limbs have been derived, I bring a 

 second which sees in the same only a special development peculiar 

 to the Dipnoi. It is this. 



As the dorsal and anal fins ^cere specializations of the median folds 

 of Amphioxus^ so the paired fins were specializations of the tioo lateral 

 folds which are supplementary to the median in completing the cir- 

 cuit of the body. These lateral folds, then, are the homologues of the 

 Wolffian ridges, in embryos of higher forms. Here, as in the median 

 fins, there were formed chondroid and finally cartilaginous rods. 

 These became at least twice segmented. The orad ones, wuth more or 

 less concrescence proximally, were prolonged inwards. The cartilages 

 spreading met in the middle line, and a later extension of the carti- 

 lages dorsad completed the limb girdle. 



If now we seek to determine the form of limb for the Protognathos- 

 tomi, that is to say, for that time for which the archipterygium in 

 its entii'ety is proposed, we should propose this. 



TTie limbs of the Protognathostomi cimsisted of a series of parallel 

 articulated cartilaginous rays. They may have coalesced somewhat 

 proximally and orad. In the ventral pair they had extended them- 

 selves mesiad until they had nearly or quite met and formed the hip 

 girdle. They had not here extended themselces dorsad. In the pec- 

 toral limb the same state of things prevailed, but was carried a step 

 further, namely, by the dorsal extension of the cartilage constituting 

 the scapidar portion, thus more nearly forming a ring or girdle. 



This theory naturally diA'ides itself into two parts, namely, the 

 derivation of the Gnathostome limb from a series of sim})le parallel 



