348 AXXALS XEW YOB.K ACADEMY OF SCIENCES 



(Fig. 4B), while the dermal elements of the shoulder-girdle (except the 

 true claYieles which disappear) are fully developed. These dermal ele- 

 ments are associated with the opercular and brancliiostegal series and 

 serve to attach the pectoral girdle to the skull; but their chief function 

 is to serve as a base for the coracoscapula. which bears the principal 

 muscles of the fin. The homology of all these elements with those of 

 the Tetrapoda is discussed below. 



In brief the t\'pical Actinopterygii while progressively emphasizing the 

 bony tissue, both in the exoskeleton and the endoskeleton, enjoyed only a 

 moderate development of the basals and radials of the paired fijis, never 

 protruded the basal elements widely from the bod}--wall and depended 

 primitively on the caudal fin as the chief accessor}- locomotive structure, 

 originally using the other fins chiefly as keels, rudders, brakes and bal- 

 ancers (E. C. Osburn, 1906). 



DIPXOI 



In this group the exoskeleton is primitively like that of the earliest 

 Crossopterygii, but it soon undergoes degenerative changes (Goodrich, 

 1909, pp. 230, 238) sinking beneath the skin and losing the ganoine and 

 cosmine layers. The endoskeleton, on the other hand, never passes 

 beyond the cartilaginous stage. 



The earliest Dipnoi resemble their contemporaries the Ehipidistia in 

 having two dorsal fins, a heterocercal tail, lepidotrichia with ganoine and 

 cosmine, and paired fins of mesorhachic or biserial type. 



The tail never attains the homocercal type : it never attains much- 

 expanded hypurals, but passes from the primitive heterocercal type 

 through intermediate stages, described by DoUo (1895), into the degen- 

 erate geph}Tocercal. In such an animal the caudal fin has not the com- 

 manding functional importance that it has in the t}'pical Actinopterygii, 

 and there is a very evident tendency to throw the function of locomotion 

 more upon the paired fins and upon an eel-like undulation of the body. 

 The Dipnoi parallel the recent Amphibia in this respect and the more 

 specialized types also show a reduction of the dermal rays. The paired 

 fins of the Devonian Dipnoi and still more of the modern Ceratodus are 

 externally very unlike the paired fins of elasmobranchs or Actinopterygii; 

 they also resemble vaguely the paddles of plesiosaurs rather than the 

 cheiroptergia of tetrapods. The modern Protopierus, however, some- 

 times uses its pectoral and pelvic fins as if they were legs, crawling about, 

 while floating in the water, on the tips of them (Dean. 1903) ; while 

 Ceratodus sometimes rests in the water (Dean, 1906), with the tips of 

 the pectorals turned downward and tonchincr the bottom. But the ability 



