GREGORY, PRESENT STATUS OF ORIGIN OF TETRAPODA 369 



An important fact in connection with the origin of the tetrapod limbs 

 is that in all known Carboniferous Amphibia and Eeptilia as well as in 

 recent nrodeles all the digits, together with their carpals or tarsals, con- 

 verge toward the ulna or fibula respectively, and that the whole carpus 

 and tarsus are obliquel}' placed with reference to the ulna and fibula. 

 This arrangement appears to be foreshadowed in the rhizodonts (Fig. 

 11) ; especially if we assume that only those radials that converge toward 

 the mesopterygium were preserved, and that the postaxial radials gave 

 rise to the vestigial prehallux and prepollex which are so widely distrib- 

 uted in the Tetrapoda (c/. Emery, 1897). 



Although an ilium and a sacral attachment to the backbone are not 

 known in fish, the readiness with which the backbone forms an attach- 

 ment with the bases of the fins in different groups of fishes, either through 

 the hypural bones, or, in the Rhipidistia, with the expanded bases of the 

 dorsal and anal fins, indicates that with the enlargement of the hind 

 limbs and pelvis the development of paired dorsal apophyses or ilia and 

 their subsequent attachment to the sacral ribs would soon follow. Indeed 

 the sacral ribs of Eryops and even of modern urodeles furnish an exam- 

 ple of the imperfect connection between the pelvis and the backbone that 

 existed in early tetrapods. 



"PROCORACOID AND CORACOID" OR "CORACOID AND METACORACOID" ? 



Notwithstanding all that has been written since Howes's paper (1887) 

 in the effort to homologize the coracoidal elements throughout the Tetra- 

 poda, investigators are still divided as to the application of these terms 

 and as to the implied homologies. Professor AYilliston, in various publi- 

 cations, inclines to the view that the true mammalian coracoid is repre- 

 sented in the primitive reptiles by the anterior one of the two coracoidal 

 elements; he accordingly calls it "coracoid," while the posterior element 

 he calls "metacoracoid" ; other investigators, however (including Huene, 

 Broom and Watson), continue to use the terms "procoracoid" and "cora- 

 coid.'^ 



With the hope of coming to some decision in the matter I have com- 

 pared the shoulder-girdle of representative Tetrapoda of all classes and 

 Professor L. A. Adams has prepared the series of figures (Figs. 12-15) 

 here published. It seems to us that Broom's observations (1899) on the 

 development of the shoulder-girdle of marsupials offer decisive evidence 

 that the true coracoid is the main element that extends down to the 

 sternum in embryonic marsupials (Fig. 12, A, B), but we regard the 

 mesenchymatous anterior element called by Broom "procoracoid" as more 

 probably homologous with the epicoracoid of the monotremes and lizards 



