474 Dr. Branislav Petronievics on the 



basal axonost as a separate postaxial radial (in this respect 

 the restoration o£ Broom is exact). 



Now I consider the posterior bifurcation of the fourth 

 axonost in our specimen as of exceptional importance for the 

 question of homologies. As the pelvic tin of Eusthenopteron 

 is far more reduced than Its pectoral tin (comp. tig. 1 of 

 pi. xvi. in Goodrich, 1902, which shows that there is no 

 fourth axonost in the pelvic fin — British Museum specimen 

 P. 6794 — and no postaxial processes), we must infer that the 

 paired fins of Eusthenopteron represent a stage far in advance 

 of that stage of the paired tins in its ancestors, which was 

 the starting-point for the evolution of the paired limbs in the 

 primitive ancestors of the Tetrapoda *. If this inference is 

 a right one, then it is not improbable that the posterior 

 bifurcation of the fourth axonost in our specimen is a remnant 

 of a more primitive stage when the fourth axonost was com- 

 posed of two separate ossifications, the paired fins of Eustheno- 

 pteron being evidently the reduced archipterygium-type of 

 Gegenbaur (a resemblance recognized by Woodward, Tra- 

 quair, and others). So that we have to conclude from this 

 evolution that the axis of the tetrapod limb runs along the 

 humerus, ulna, ulnare, and between the fourth and fifth finger f 

 (comp. text-fig. 2, in which some further hypothetical homo- 

 logins have been indicated). This conclusion, as one sees,' 



* This conclusion is confirmed also by the skull, which in Eustheno- 

 pteron is simpler than in the more primitive Osteolepidae, whose paired 

 fins are also less reduced \comp. the tins of Megalichthys figured by 

 Ed. D. Wellburn in his paper " On the Genus Megalichthys" in Proc. 

 Yorkshire Geol. & Polytechnic Soc. vol. xiv., 1900). 1 may add in this 

 connexion that the skull of Osteolepis may be considered to approach 

 nearer to the Stegocephalian skull than is shown by the restoration of 

 Pander (comp. Chr. II. Pander, ' Ueber die Saurodipterinen, &c.,' 1860, 

 pi. i. figs. 8 & 9), lately reproduced by Gregory (comp. Gregory, 1915, 

 tig. 2, A, B). Pander's restoration was founded on the specimen of 

 Osteolepis microlepidotus figured by him in pi. i. fig. 1 ; but tig. 4 on the 

 same plate represents a specimen in which all the three characteristic 

 bones of the Stegocephalian skull (supratemporal, intertemporal, po.st- 

 orbital) are present. 



t The pectoral fin of Sauripterus taylori (figured and restored by 

 Gregoiy, 1915, plate iv. and tig. 9) does not militate against this supposi- 

 tion. This fin, less reduced than that of Eusthenopteron, has three 

 elements attached to the third axonost, so that these three elements may 

 correspond with the three digits on the ulnar side of the tetrapod limb. 

 As the two outer of these three elements have almost the same length, 

 it may well be supposed that the axis runs between the two (and not 

 along the outer one alone, as Gregory hypothetically supposes — comp. 

 Gregory, 1915, p. 360). I should mention that the first to emphasize 

 the resemblance of the Suwij>terus-Rn with the tetrapod limb was its 

 discoverer, James Hall himself (comp. J. Hall, ' Geology of New York,' 

 part iv. 1843, p. 282). 



