5 I2 



The Class Elasmobranchii or Shark-like Fishes 



Woodward, and several different suggestions have been recorded. 



If with Gegenbaur we regard the paired fins as derived 

 from the septa between the gill-slits, or with Kerr regard them 

 as modified external gills, the whole theoretical relation of 

 the parts is changed. The archipterygium of Pleuracanthus 

 would be the nearest approach to the primitive pectoral limb, 

 and from this group and its allies all the other sharks are 

 descended. This central jointed axis of Pleuracanthus is re- 

 garded by Traquair as the equivalent of the metapterygium 

 in ordinary sharks. (See Figs. 44, 45, 46.) 



According to Traquair: "The median stem [of the archip- 

 terygium], simplified, shortened up and losing all its radials 

 on the postaxial side, except in sometimes a few near the tip, 

 becomes the metapterygium, while the mesopterygium and 

 propterygium are formed by the fusion into two pieces of the 

 basal joints of a number of preaxial radials, which have reached 

 and become attached to the shoulder-girdle in front of the 

 metapterygium . ' ' 



According to Dr. Traquair, the pectoral fin in Cladodus 

 neilsoni, a shark from the Coal Measures of Scotland, is "appar- 

 ently a veritable uniserial archipterygium midway between the 

 truly biserial one of Pleuracanthus and the pectoral fin of ordi- 

 nary sharks." Other authors look on these matters differently, 

 and Dr. Traquair admits that an opposite view is almost equally 

 probable. Cope and Dean would derive the tribasal pectoral 

 of ordinary sharks directly from the ptychopterygium or fan- 

 like fold of Cladoselache, while Fritsch and Woodward would look 

 upon it as derived in turn from the Ceratodus-like fin of Pleura- 

 canthus, itself derived from the ptychopterygium or remains 

 of a lateral fin-fold. 



If the Dipnoans are descended from the Crossopterygians, as 

 Dollo has tried to show, the archipterygium of Pleuracanthus 

 has had a different origin from the similar-appearing limb of 

 the Dipnoans, Dipterus and Ceratodus. 



In such case the archipterygium would not be the primi- 

 tive pectoral limb, but a structure which may have been inde- 

 pendently evolved in two different groups. 



In the view of Gegenbaur, the Crossopterygians and Dip- 

 noans with all the higher vertebrates and the bony fishes would 



