18/6.] PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 0.') 



cisely stated in his c Grundriss der vergleichenden Anatomic,' L 87*4, 

 j). 493, in the following words : — 



" The very various forms of the skeleton of the free limbs are 

 deducibie from a fundamental form of which only a few instances 

 remain, and which, as representing the first and lowest condition of 

 the fin-skeleton, I term the archipterygium. This is represented by 

 a jointed cartilaginous stem, articulated with the pectoral arch and 

 giving attachment, on each side, to a series of likewise jointed 

 pieces, the radii. The whole structure, resembling a pinnate leaf, 

 is singularly like the supporting apparatus of many Selachian gills, 

 and thus throws a gleam of light upon the phylogeny of the limbs. 



" Ceratodus presents this form of fin-skeleton, which was perhaps 

 usual among the Crossopterygidce, at present represented only by 

 Polypterus. The biserial rays of the fin undergo different modifica- 

 tions. Among the Dipnoi the medial [postaxial] rays are retained 

 in the form of thin rods of cartilage ; while in the Selachians the 

 lateral [prseaxial] rays attain a considerable development and consti- 

 tute the greater part of the massive fin-skeleton. Of the medial 

 [postaxial] rays but few remain, though they are sufficiently distinct 

 to sanction the assumption of a former more extensive biserial 

 arrangement of rays on the stem of the fin." 



The metapterygium Gegenbaur considers to answer to the axial 

 skeleton of the archipterygium. The propterygium is formed by 

 the union of the proximal praeaxial fin-rays. The mesopterygium is 

 formed by a certain number of the succeeding praeaxial fin-rays. 



The only part of this interpretation with which I can agree is the 

 determination of what Gegenbaur names the propterygium as the 

 representative of the proximal prseaxial fin-ray or rays in most cases, 

 but not in Chimcera, and probably not in Notidanus. 



In my judgment, the mesopterygium of Gegenbaur is the proximal 

 piece of the axial skeleton, which constantly retains its primary 

 articulation with the pectoral arch. His propterygium represents 

 the proximal praeaxial fin-ray, and his metapterygium the proximal 

 postaxial fin-ray in almost all cases ; and the ichthyopterygium, as 

 the typical fish-fin may be termed, differs from the archipterygium 

 not by the more or less complete suppression of the postaxial rays, 

 but by the general abbreviation of the whole skeleton and the gradual 

 connexion of more or fewer fin-rays (parameres) with the pectoral 

 arch. 



In the effectual discharge of the function of the fish's fin, increase 

 of breadth is needed ; and this increase of surface is obtained by the 

 gradual approximation of more and more lateral elements of the 

 archipterygium to the shoulder-girdle. 



Professor Gegenbaur has extended his theory of the limbs to the 

 higher Vertebrata. He conceives that the axis of the archipterygium 

 (which he considers to be the homologue of the metapterygium of 

 the Selachian) is represented by the series of bones which is formed 

 bv the humerus, the radius, the radial segments of the carpus, and 

 the radial digit or pollex ; while the ulna, the radial segments of the 

 carpus and the ulnar digit, the other carpal bones, and the fourth, 



