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



mesomere and the swollen basal piece referred to ; and the whole 

 condition of the parts is such as would have resulted did the proximal 

 mesomere and its related lateral elements represent a shortened-up 

 ^metapterygium. 



Beyond this, the pelvic member of Ceratodus differs most con- 

 spicuously from that of the Plagiostomes and Osteichthyes in the 

 presence of an elongated mesopterygium. Balfour first showed 

 (1, pp. 666-7) that the development of the pelvic fin of the Shark 

 is arrested at a comparatively early stage. He and subsequent 

 writers regard the enlarged preaxial ray, which Huxley would hold 

 to represent the mesopterygium, as the propterygium. He has also 

 called attention to the fact that the mesopterygium is not there re- 

 presented ; and the only anticipation of that structure forthcoming 

 among the Plagiostomes, known to me, is the comparatively insig- 

 nificant one described by Haswell (16. p. 23, pi. i. fig. 3) for 

 Heptanchus indicus, 



VI. On the Homologies of the Chimcsroid Fin-skeleton, as compared 

 with that q/" Ceratodus. 



Huxley, discussing the morphology of the Chimseroid pectoral fin, 

 insists (19, pp. 52-53) upon the close relationship which it bears to 

 that of Ceratodus. The former fin is, as is well known, supported 

 upon two basal elements, both of which are in intimate connexion 

 with the pectoral girdle. The postaxial of these is held by all to 

 represent the metapterygium. 



As to the preaxial cartilages: — Huxley (19, pp. 52— 53), whose 

 view demands that the mesopterygium " constantly retains its primary 

 articulation with the pectoral arch," completely reverses Gegenbaur's 

 determination (9, p. 145), and regards the smaller basal one as 

 the mesopterygium, and the larger ray-like distal one which it bears 

 as the propterygium. Mivart, on the other hand, insists (21, 

 p. 478) on the absence of the mesopterygium, and regards both pre- 

 axial elements as homologous with the propterygium. Comparison 

 of the fin of GhimcBra with that of the Selachians, as represented 

 in Hexanchus, appears to me to warrant his view. 



It is interesting, here, to recall Huxley's remarks upon the 

 metapterygium, when dealing with Chimcera, Having asserted the 

 belief that the metapterygium of Notidanus is "formed by the 

 coalescence of the axial ends of the postaxial rays," he goes on to 

 say (p. 53), " the metapterygial cartilage cannot, in Scyllium, at 

 the same time represent coalesced postaxial rays, as the analogy of 

 Notidanus would suggest, and the second joint of the axial skeleton 

 as the analogy oi Chimcera .... indicates." Did the mesopterygium 

 exist in Chimcera in the form so constant among the Plagiostomes 

 — that of a fusion of the basal ends of the rays interposed between 

 the pro- and metapterygia — he could, in comparing the Chimseroid 

 and Ceratodus fins, only have come to the conclusion formed by 

 me (p. 15), in describing the second pectoral mesomere of the latter. 



The pelvic fin of the Cbimseroid is in an exceptionally interesting 



