GREGORY, PRESENT STATUS OF ORIGIN OF TETRAPODA 351 



the endoskeletal base consists of a single proximal element, analogous 

 respectivel}' with the humerus and femur. Other proposed homologies 

 of the more distal elements with the forearm, carpus and digits have been 

 assumed, but lack adequate evidential support. 



The shoulder-girdle of the Devonian Dipnoi evidently included dermal 

 elements ("infraclavicles," ^^clavicles," etc.) and underlying cartilaginous 

 elements, which, as Smith Woodward sa3'S (1898, p. 63), ^''seem to have 

 been always too slightly calcified for preservation.^' But from the re- 

 semblance of the pectoral limb of Diptems to that of Ceratodus, it is 

 highly probable that the endoskeletal elements in the Devonian forms 

 were fundamentally similar to that of Ceratodus. 



The accompanying figures (Fig. 5), drawn from a specimen of Cera- 

 todus forsteri by Professor L. A. Adams, show that in the modern genus 

 the chief cartilaginous element, which is supposed to be homologous with 

 the coracoscapula of the Tetrapoda, is of large size, and includes a short 

 ascending ramus corresponding to the scapula, a tripartite, protuberant 

 articular pedicle, and a long forward-and-inwardly projecting coracoid. 

 This undivided element is firmly adherent to the antero-internal surface 

 of the conjoined dermal elements first named by Gegenbaur clavicle 

 ("infraclavicle'' of early writers) and cleithrum ("clavicle"). A small 

 median ventral cartilage (m. v. c.) unites the opposite clavicles, while a 

 broad dermal supracleithrum connects the pectoral girdle with the post- 

 temporal. 



This shoulder-girdle is apparently more primitive than that of the 

 primitive Tetrapoda in the slight dorsal extension of the scapula, in the 

 unexpanded form of the coracoid, and especially in the protuberant form 

 of the articular region; also, there are no centers of ossification tending 

 to divide the coracoscapula mass into distinct elements. The dermal 

 elements, especially the cleithrum and supracleithrum, are of large size, 

 whereas in Tetrapoda they are much reduced. This shoulder-girdle is 

 on the whole nearer to the tetrapod type than that of any other existing 

 fish. 



The pelvic girdle of Ceratodus also differs from that of typical fishes 

 and approaches that of the primitive Tetrapoda in having a stout carti- 

 laginous base, perhaps homologous with the ischio-pubis. The "pre- 

 pubic" and '^epipubic" processes are of doubtful homology. No dorsal 

 process analogous with the ilia and extending up from the acetabulum 

 toward the backbone is present. 



In brief the more specialized Dipnoi have paralleled the urodele Am- 

 phibia in the reduction of the dermal rays, in the degeneration of the 

 scales, in the non-development of a powerful homocercal tail, and in the 



