GREGORY, PRESENT STATUS OF ORIGIN OF TETRAPODA 349 



to do this ranks very far below the ability to support the weight of the 

 body on the paired limbs without the buoyant effect of the water, as in 

 the Tetrapoda. 



In Ceratodus the preaxial or upper border of the pectoral fins is be- 

 lieved to be serially homologous with the ventral border of the pelvics. 

 Braus (1901, p. 165) states that the, earliest anlage of both fins are 

 horizontal, that the nerve entrance of the pectoral is on the lower, or 

 ventral, surface, while that of the pelvic is on the dorsal surface. Ac- 

 cording to this view the preaxial border of the pectoral has been rotated 

 upward as in most Actinopterygii while in the pelvics the preaxial border 

 has been rotated downward. Schneider (1886, quoted by Howes, 1887, 

 p. 12) held that "die Seitenstrahlen der dorsalen und ventralen Halfte 

 der [pectoral and pelvic] Flossen sind ungleich," and that the "Seiten- 

 strahlen der dorsalen Halfte der einen Flosse entsprechen derjenigen der 

 ventralen Halfte der anderen." Goodrich (1909, p. 244) says that "when 

 at rest the preaxial margin of the pectoral fin is borne upward; the re- 

 verse is the case with the pelvic fin.'' But Howes (1887, pi. II) has de- 

 scribed a pelvic fin of Ceratodus in which this reversal has not been 

 effected and he records so many irregularities in the structure of both 

 pectoral and pelvic fins that the condition of reversed homology seems to 

 be incompletely attained. 



Many investigators {cf. Keith, 1912, p. 418) have sought to show that 

 in man there is a somewhat similar condition of reversed homology in 

 the borders of the pectoral and pelvic extremities ; but according to H. H. 

 Wilder (1909, p. 245), the evidence of embryological history as well as 

 of comparative anatomy lends strong support to the opposite view that 

 in the Tetrapoda the preaxial or anterior border of the pectoral extremity 

 is homologous with the preaxial border of the pelvic extremity, a con- 

 clusion which is further strengthened by the general correspondence in 

 the arrangements of the elements of the manus and pes in the most primi- 

 tive Tetrapoda of the Permocarboniferous, such as Eryops, Stereosternum 

 and the cotylosaurs. 



If the latter view be correct the reversed homology of the borders of 

 the pectoral and pelvic fins in Ceratodus is a point of marked difference 

 from the Tetrapoda, which may have been acquired only by the later 

 Dipnoi. 



As the paired fins of even the oldest Dipnoi are already biserial in 

 form, there is no direct palseontological evidence as to their origin. In 

 the allied group of Ehipidistia, however, we get several hints as to the 

 origin of the "archipterygial" type. Osteolepis, which I regard as the 

 most primitive member of the group, on account of its skull structure, 



