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



shoulder-girdle may have been due to a displacement of the same 

 by two rays. Should this be so, the metapterygium must there 

 have disappeared, as from the Ceratodus pelvic fin, under the 

 corresponding enlargement of the mesopterygial plate. The only 

 alternative view possible is that the metapterygium does repre- 

 sent thnt of the Elasmobranchs. If this be so, comparison of the 

 pectoral fin of Pohjpterus (fig. 11) with that of the Plagiostomes, as 

 represented in Scyllium, where the mesopterygium is relatively small, 

 would seem to show that the loss of connexion between the meta- 

 pterygium and its ravs has been to no small extent due to a 

 displacement of the latter by the elongation of the expanding 

 mesopterygium, no less than by the simplification in structure of the 

 metapterygium itself. The last step in the former process would 

 appear, indeed, to be retained in the living Polypterus (** fig. 11). 

 In the absence of embryological data further discussion of this 

 difficulty would be fruitless. It is much more pertinent to observe 

 that in both Ceratodus and Polypterus the initial step in the modi- 

 fication has been, in any case, one of elongation of the mesoptery- 

 gium, and evidence has beeu adduced to show that in the Dipnoi (if 

 not in Polypterus also) the metapterygium has been thereupon 

 reduced and finally suppressed. The only traces of either it or 

 its rays yet recorded in the Ceratodus pelvic member are forthcoming 

 in fins whose postaxial parameres are more numerous and less 

 specialized than is generally the case. If this simplification in 

 structure of the most specialized portion of the pelvic fin-skeleton 

 is, as I have attempted to show, reversionary to a condition which 

 has been lost, the characters of those elements which reappear 

 under the simplification, when compared with those to which they 

 most nearly correspond in the pectoral fin, go far towards bearing 

 out the presumed origin of the Ceratodus fin from a primarily 

 expanded predecessor. 



V. On the Morphology of the Axis of the Ceratodus Fin. 



The entire axis of the Ceratodus fin is held by Huxley (19) 

 Balfour' (I), and v. Eauteufeld (22) to represent an elongated 



the two specimeus examined by me. It has been figured by Wiedertheim (29, 

 p. 199), but I have been unable to find a description of it. I think it not 

 improbable that it may have been deriTed from the mesopterygium, the closely- 

 related lower anterior end of which may (as "U'iedersheim has shown) insert 

 itself between the supposed propterygium and the marginal rays. In the 

 absence of embryological data further discussion of it would be useless. 



^ Baur has recently called attention (3, p. 6) to the fact that Gervais has 

 priority over Humphrey in the enunciation of the hypothesis that the paired 

 fins are dismembered portions of a lateral fold. Gervais writes (12; : — "Si Ton 

 considere que les rayons des nageoires impaires des poissons ont une analogie 

 incontestable avec ceux dout la reunion formes les nageoires paires des memes 

 animaux, c'est a dire leurs membres yeritables, on est naturellement conduit a 

 se demander s'ils ne seraient les homologues de ces demiers, et si I'etat d'isole- 

 ment dans lequel ils restent les unes par rapport aux autres, ne resulterait pas 

 de ce que chacun d'eux ne conserve pas completement ses rapports avec celui 

 des segments osteodesmiques dont il est tributuire. Alors on pourrait les 



