^322 ANNALS NEW YORK ACADEMY OF JSC1E\CE>^ 



the tetrapods on the other all the characters which separate the Gnatlio- 

 stomata from the Agnatha. But they have advanced beyond the elasmo- 

 branchs toward the tetrapod type in many particulars, especially : ( 1 ) in 

 the a23pearance of true bone cells both in the exo- and endoskeletons ; (2) 

 m the formation of scales, dermal rays and dermal plates, having in the 

 most primitive forms an outer cosmine layer, a middle zone of vasodentine 

 and basal layers of isopedine ; ( 3 ) the formerly continnons dermal cover- 

 ing of the head and trunk is now fragmented into the dermal plates of 

 the skull, of the l)ranchial region (operculars, gulars, etc.) and of the 

 pectoral region; (4) the spiracular cleft is usually closed. 



Most of the known Osteichthyes are excluded from the ancestry of the 

 Tetrapoda by various specializations, either of the exoskeleton or of the 

 median and paired fins as noted below. 



In the Actinopterygii, a sufficiently full morphological series enables 

 us to follow the changes in the group, beginning with the very generalized 

 Devonian Clieirolepis and culminating in the most specialized types of 

 modern teleosts. In the most primitive of the series (Palseoniscidae) the 

 elements of the skull-roof may be in general homologized with those of 

 the most primitive Crossopterygii, although the details are quite different ; 

 there is, however, no special resemblance to the skull of the earliest 

 Stegocephali, except in so far as the elements of the skull-roof include 

 paired frontals, parietals, pterotics and other paired elements. The brain 

 also is diversely specialized in the surviving forms. 



DIPNOI 



A closer structural approach to the tetrapod type is attained in this 

 group. In the brain the Dipnoi have retained the well-developed olfac- 

 tory lobes and eerebra, whicli are requisite for the ancestral tetrapod. 

 They also have a functional lung and, as Kellicott (1905) lias shown, 

 the venous system presents close ontogenetic resemblances to the urodele 

 type. The larvse and embryos of the Dipnoi, as well as of the Crossop- 

 ter3^gii, reveal further well-known strikins^ resemblances to the urodeles, 

 and, as noted below, there are nicinv featuies of the locomotive ors:ans 

 and mode of locomotion in Dipnoi that foreshadow the conditions in 

 Tetrapoda. While much of this might be ascribed to convergence, it all 

 implies a similarity in the ^'potential of evolution,^' that is, of structural 

 ])Ossibilities, in the forerunners of these groups. 



The known Dipnoi are all excluded from direct ancestry to the 

 Amphibia by the specialized character of the dentition, including the 

 formation of complex radially arranged tritoral plates on the roof of 



