24 PROF. G. B. HOWES ON THE SKELETON AND [Jan. 18, 



If the above suggestion should prove to have veeight, the condition 

 of the basal parts of the Polypterus fin, in vfhich the mesopterygiura 

 is in no way in connexion with the shoulder-girdle, can only be a 

 lowly one. 



It may not be inappropriate here to call attention to the conception 

 lately put forward by Baur (2, p. 663; concerning the morphology 

 of the cheiropterygium. He returns to Gegenbaur's first position, 

 and maintains that the limb of the land-animal has been derived 

 directly from the ichthyopterygium. In that case the Ceratodus fin, 

 as it stands, can only represent the initial phase in a line of modifi- 

 cation of the ichthyopterygium, culminating in Protopterus (to 

 include Lepidosiren. Of. Ayers, Jenaische Zeitschr. vol. xviii. 

 p. 479, 1885, and Schneider, op. cit.), 



Davidoff claims that the Ceratodus pelvic fin (7, p. 127) " trotz 

 der Einfachheit des Gauzeu, sich bedeutend komplicirter gestaltet, 

 als bei den friiher bearbeiteten Fischen." He uses the words in a 

 physiological sense, it is true, but that in face of his concluding 

 statement that (p. 160) "das Endergehnis aber besteht darin, dass 

 von der Ceratodus-Extremitat sich diejenige der Haie ohne Schwier- 

 igkeiten ableiten lasst." This is, in my opinion, far from proven. 



VII. Conclusions. 



1 . That the characters of the skeleton of the Ceratodus paired 

 fins are inconstant, except for those of the preaxial parameres of the 

 pectoral fin and the basal mesomere of both pectoral and pelvic fins. 



2. That a metapterygium is always present in the fore limb, in a 

 reduced condition and usually confluent with the second mesomere. 



3. That traces of what appears to represent a metapterygium are 

 occasionally to be met with in the hind limb, under conditions which 

 point to atavism. 



4. That the basal mesomere of the Ceratodus fin may conceivably 

 have been derived from the metapterygium. 



5. That the structural features of both paired fins of the Chimae- 

 roids are identical, and characterized by the absence of a meso- 

 pterygium. 



6. That the paired fins of the Plagiostomes and Dipnoi have, in 

 all probability, arisen independently from a type of fin most nearly 

 represented by that of the living Chiniseroids. 



7. Proven incidentally. — That the basal cartilage of the Cestracion 

 pectoral fin, usually regarded as the mesopterygium, is a compound 

 of the pro- aud mesopterygia of other Plagiostomes. 



VIII. List of Authorities referred to. 



1. Balfour, F. M. — On the Development of the Skeleton of the 



Paired Fins of Elasmobranchii. Proc. Zool. Soc. 1881, p. 656. 



2. Baur, G. — Ueber das Archipterygium und die Entwickluug des 



Cheiropterygiums aus dem Ichthyopterygium. Zool. Anzeiger, 

 vol. viii. p. 663 (188?). 



