20 PROF. G. B. HOWES ON THE SKELETON AND [Jail. 18, 



the Ceratodus fin has arisen as an elongation of a primarily expanded 

 predecessor, that such a fusion of the approximated ends of the rays 

 could have resulted from that simple process. I am of oj)inion that 

 the distal portion of the mesopterygium of Ceratodus has arisen on an 

 elongation of a pre-existing plate, hy a process such as is seen at its 

 earliest phase in Pohjpterus. 



A difficulty, however, arises with respect to the basal mesomere 

 of Ceratodus, which, if it represents the proximal end of the meso- 

 pterygium, differs from that of all other fishes in forming (in the 

 pectoral fin, at any rate) the sole support for the base of the fin. 

 Gegenbaur, who has paid considerable attention to this matter, at 

 first acquiesced (10) in Huxley's belief in the close relationship 

 between Polypterus and the Dipnoi. Commenting upon the pectoral 

 fin of the former animal, he writes (p. 138) " ausser den Selachiern 

 bei denen die zweieilige Form des Archipterygium in die einzeilige 

 iibergeht, besitzt vielleicht nur noch Polypterus unter den lebenden 

 Ganoiden das primare Archipterygium im Flossenskelete." He goes 

 on to advance the view that the fin of Polynterus represents a 

 shortened-up derivative of the Ceratodus type ; but finally he reverts 

 to his original position, holding, chiefly on account of the loss of 

 connexion between the mesopterygium and shoulder- girdle, and of 

 the great structural difference between the pectoral and pelvic fins, 

 that (p. 140) "demnach kann ich das genannte Skelet von Polypterus 

 nicht uumittelbar auf das primare Archipterygium beziehen, sonderu 

 leite es, wie jenes der anderen lebenden Ganoiden, von der secund- 

 aren, nur eine Reihe von Radieu besitzeuden Form ab." 



The pelvic fin of Ceratodus appears, at first sight, to be ex- 

 ceptional in the possession of a well-developed mesopterygium. 

 Davidoff has brought forward good evidence to show that the 

 element hitherto regarded among Ganoids and Teleostei as the 

 pelvic girdle is (5. p. 125, and 6. p. 433) homologous with the 

 basal piece of the Ceratodus fin, and he terms it the basal segment 

 of the metapterygium. He has shown good reason for believing that 

 the true pelvis is seen for the last time among the Osteichthyes in 

 Polypterus, where it is represented by two or three vestigial carti- 

 lages {cf. 6. p. 462, pi. 21, and Wiedersheim, 28) lying imme- 

 diately in front of the applied ends of the basal pterygia of ojiposite 

 sides. Upon careful consideration, I am disposed to accept his 

 interpretation as it applies to the Ganoids and Teleostei, but I am 

 more dubious about it as applying to the Dipnoi. 



Examination of either of the paired fins of Ceratodus in relation 

 to the limb-girdle appears, at first sight, to favour Huxley's 

 view that the whole fin-axis answers to the Selachian meso- 

 pterygium. If this be so, that element must, iu elongating, have 

 carried down with it the metapterygium, and the propterygium if 

 present. 



Balfour, criticizing Huxley's view that the basal mesomere is the 

 proximal piece of the axial skeleton of the limb of Ceratodus, says 

 (1, p. 669), "The entirely secondary character of the mesoptery- 

 gium and its total absence in the young em.bryo Scyllium appear to 



