52 MR. F. M. BALFOUR ON THE SKELETON 



the rays of the skeletons in many unpaired fins. He sums 

 up his views in the following way ■} 



" As the dorsal and anal fins were specialisations of the 

 median folds of Awphioxus, so the paired fins were speciali- 

 sations of the two lateral folds which are supplementary to 

 the median in completing the circuit of the body. These 

 lateral folds, then, are the homologues of "Wolflfian 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 concrescence 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 themselves 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 Thacher's theories which I 

 cannot accept is the derivation of the folds, of which the 

 paired fins of the Vertebrata are supposed to be specialisa- 

 tions, from the lateral folds of Amphioccus ; and Thacher 

 himself recognises that this part ot his theory stands on 

 quite a difi'erent footing to the renaainder. 



Not long after the publication of Thacher's paper, an 

 important memoir was published by Mivart in the 'Trans- 

 actions'' of the Zoological Society.^ 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 centripetal chondrifications or 

 calcifications, and so be genetically exoskeletal.^ 



Mivart^s investigations and the majority of his views were 

 independent of Thacher^s memoir ; but he acknowledges that 

 he has derived from Thacher the view that pelvic and pec- 



» Loc. cit., p. 298. 



' St. Georjie Mivart, " On the Fins of Elasmobrancbii," ' Zoological 

 Trans.,' vol. x. 



3 Mivart used the term exos.keletal in an unusual and (as it appears to 

 me) inconvenient manner. Tiie term is usually applied to dermal skeletal 

 t-lructures ; but ihe skeleton ot the limbs, with which we are here con- 

 cctuid, is uiidoubledlj not of- this nature. 



