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



were beginning to unite. The propterygium {pt.) is seen to com- 

 pose fully the anterior third of the whole mass, its base being 

 made up of two segments, the proximal one of which contributes 

 nearly half the articular cup, entering at least as fully into the 

 formation of the same as does its representative in Scyllium (cf. 

 Huxley, 19, p. 48, fig. 10). In the adult fin (fig. 10) the original 

 boundary line between the pro- and mesopterygia is represented by 

 a groove indicated in Huxley's figure (here reproduced) by a 

 dotted line. 



The pectoral fin of Oestraeion is thus shown to conform to the 

 Selachian type, being identical most nearly with that as represented 

 by Acanthias (cf. Gegenbaur, 9, pi. 9. fig. 4, and Mivart, 21, 

 pi. 77. fig. 2). Cestracion must, on the evidence now forthcoming, 

 relinquish its position in the series established by Huxley ; it must, 

 to say the least, change places with Notidanus. The main articu- 

 lation of the fin of ihe last-named fish is established, as is well known, 

 through the agency of the mesopterygium. Gegenbaur originally de- 

 scribed a basal preaxial bar in Notidanus, and he homologizes it (9, 

 p. 140) with his propterygium. This enters, if anything, more fully 

 into the articulation with the shoulder-girdle than does its representative 

 in Cestracion. There is connected with its distal end, in Hexachus, 

 a smaller piece (c/'. Gegenbaur, op. cit. pi. 9. fig. 1) which appears 

 to represent the second segment of Cestracion, reduced, as an 

 outcome of the great expansion of the front border of the meso- 

 pterygium. Mivart has suggested (21, p. 444) that the mesoptery- 

 gium represents a coalescence of the pro- and mesopterygia ; but I 

 am inclined, upon careful examination of the specimen under my 

 hand, to dissent from that view. 



The metapterygium of Notidanus enters into a feeble but definite 

 connexion with the pectoral arch, such as is not the case in Cestra- 

 cion. That the fins of these two genera differ from those of some 

 of the Sharks is indisputable, but they do so to an insignificant 

 degree, incapable in itself of supporting the " unibasal " type ; that, 

 in face of the facts here adduced ', rests upon an insufficient basis. 



Returning now to Ceratodus ; the lobe which, in the pectoral fin, 

 carries the 3-5 j)roximal postaxial parameres (figs. 5 and 6 mt.) is, 

 as has been stated previously, marked off from the adjoining meso- 

 mere by a deep furrow. 



Giinther observes (14, p. 532) that the conjoined mass shows, 

 in "horizontal section," lines of "the former divisions" into what 

 he holds to correspond to the " three carpals " (pterygia) of most 

 Plagiostonies. This has been denied by Huxley (19, p. 47). Set- 

 ting aside this difference for the moment, I desire to call attention 

 to the similarity of the furrow described above to that which 



^ Wlien, moreover, it is considered that in the pectoral fin of Polypfenis, 

 which Huxley relegates to the " tribasal '" category, the mesopterygium is (as 

 Gegenbaur pointed out, 10, p. 139) excluded from articulation with the Umb- 

 girdle, the statement that (p. 55) ''the mesopterygium is the proximal piece of 

 the axial skeleton, which constantly retains its primary articulation with the 

 pectoral arch," must needs be modified. 



