SKELETON OF THE PAIRED FINS OF ELASMOBRANCHS. 715 



many unpaired fins. He sums up his views in the following 

 way 1 : 



"As the dorsal and anal fins were specializations of the 

 median folds of Amphioxits, so the paired fins were specializa- 

 tions of the two lateral folds which arc supplementary to the 

 median in completing the circuit of the body. These lateral 

 folds, then, are the homologues of 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, with more or less con- 

 crescence proximally, were prolonged inwards. The cartilages 

 spreading met in the middle line ; and a later extension of the 

 cartilages dorsad completed the limb-girdle. 



" The limbs of the Protognathostomi consisted of a series of 

 parallel articulated cartilaginous rays. They may have coalesced 

 somewhat proximally and orad. In the ventral pair they had 

 extended themselves mesiad until they had nearly or quite met 

 and formed the hip-girdle ; they had not here extended them- 

 selves dorsad. In the pectoral limb the same state of things 

 prevailed, but was carried a step further, namely, by the dorsal 

 extension of the cartilage constituting the scapular portion, thus 

 more nearly forming a ring or girdle." 



The most important point in Thacker's theories which I can- 

 not accept is the derivation of the folds, of which the paired 

 fins of the Vertebrata are supposed to be specializations, from 

 the lateral folds of AmpJiioxns ; and Thacker himself recognizes 

 that this part of his theory stands on quite a different footing to 

 the remainder. 



Not long after the publication of Thacker's paper, an im- 

 portant memoir was published by Mivart in the Transactions 

 of this Society 2 . The object of the researches recorded in this 

 paper was, as Mivart explains, to test how far the hard parts of 

 the limbs and of the azygos fins may have arisen through cen- 

 tripetal chondrifications or calcifications, and so be genetically 

 exoskeletal 3 . 



1 Loc. cit. p. 298. 



' 2 St George Mivart, " On the Fins of Elasmobranchii," Zoological Trans. Vol. x. 

 3 Mivart used the term exoskeletal in an unusual and (as it appears to me) incon- 

 venient manner. The term is usually applied to dermal skeletal structures ; but the 



46 2 



