GREGORY, PRESEXT STATUS OF ORIGIN OF TETRAPODA 353 



assumption of an eel-like habitus. On the other hand the pectoral and 

 pelvic girdles of the more prinutiye Dipnoi appear to preserve certain 

 characters which may be truly homologous with those of the ancestral 

 Tetrapoda. The "archipterygia" of Dipnoi are primitive only in name, 

 although evolved in their essential features at an early epoch. That such 

 ^"arcliipterygia'' are ancestral to the cheiropterygia of tetrapods is un- 

 proved and rather unlikely. The single proximal basal pieces of the 

 paired fins may possibly, however, be homologous with the humerus and 

 radius respectively; although it is quite conceivable that here too the 

 resemblances may be only homoplastic and due to similar concrescence 

 of several originally separate basal pieces. 



CROSSOPTERYGII 



The existing Polypterus has no doubt departed widely from the primi- 

 tive Devonian Crossopterygii, both in the exoskeleton and in the endo- 

 skeleton and has paralleled the Actinopterygii in many respects. Among 

 its progressive characters may be noted : the replacement of the notochord 

 by ossified centra, the development of osseous neural and hsemal arches 

 and hypural bones (the latter, however, being arrested in development), 

 the reduction and ossification of the coracoscapula and its division into 

 two elements (Fig. 6), the reduction of the radials and basals, the fusion 

 of the basals of the pelvic fins to form a pelvis, which is ossified. And 

 Polypterus has also become highly ichthyized in the brain and many other 

 characters of the soft anatomy. Among its peculiar or aberrant special- 

 izations are to be reckoned the gephyrocercal tail, the subdivided dorsal 

 and the multiplication of the radials in the pectoral fin. 



The proximal elements of the pectoral fin are often compared with the 

 pro-, meso- and metapterygia of sharks ; the so-called "pro-" and "meta- 

 pterygium'^ forming a Y-shaped articulation with the scapulo-coracoid : 

 the ovoid "mesppter3'gium^^ lying in the middle of the fleshy lobe of the 

 fin; but this arrangement may rather represent a modification of the 

 Eusthenopteron and Sauripterus t}'pe of fin described below. Possibly 

 the "metapterygium" and "propterygium'^ may be homologous with the 

 radius and ulna of Sauripterus; the fused "mesopterygium" may repre- 

 sent the fused mass of radials, or parameres, which in Sauripterus con- 

 verge toward the central axis. The single proximal piece (true meso- 

 pterygium of Sauripterus) in Polypterus has either been lost or has per- 

 haps fused with the coracoscapula. Indeed the whole coracoscapula in 

 the larval Polypterus {cf. Goodrich, 1909, p. 296) bears a puzzling re- 

 semblance to the humerus of Sauripterus. 



At any rate the pectoral of Polypterus is further removed from the 



