GREGORY, PRESENT STATUS OF ORIGIN OF TETRAPODA 325 



Dipterus skull-top certainly includes the same elements as those of Scau- 

 menacia, the chief difference being that these elements are smaller and 

 that there are a number of other small elements in the supraorl)ital- 

 occipital series, which may later have united with adjacent elements. 

 The Dipterus skull-top also makes a distant approach to the primitive 

 rhipidistian type seen in Osteolepis, in so far as it retains lines of sensory 

 pits running longitudinally above the orbits and extending back to the 

 transverse nuchal commissure ; both skulls also have a circumorbital 

 series, a supraorbital-occipital series, a short rostrum, paired frontals, 

 parietals and other paired elements, the chief difference being that in 

 Dipterus there is a prominent median preparietal and that the nuchal 

 plates are more closely united with the occiput. 



The conclusion that Dipterus is the most primitive of all the Dipnoi 

 is further supported by the remarkable agreement in histological struc- 

 ture of the dermoskeleton of Dipterus and that of the crossopterygian 

 Osteolepis as shown in Pander's superb sections (1858, Tab. 5, Figs. 1, 

 17; 1860, Tab. 5, Figs. 1-S, 22). In both cases the dermocranium con- 

 sists of a thin outer layer of ganoine, which covers the cosmine layer, 

 underneath this is a vascular layer and on the bottom a stratified hori- 

 zontal layer. This histological pattern is also repeated in the scales of 

 both Dipterus and Osteolepis, and to my mind is of great weight in indi- 

 cating the common origin of the Dipnoi and Crossopterygii. Add to that 

 the facts : that in both groups the earliest members have the paired fins 

 of the mesorhachic or biserial tvne ; that both have two dorsal fins ; that 

 both have a heterocercal tail provided with similar dermal rays, and we 

 have a case for the common origin of the Crossopterygii and Dipnoi, 

 which is further strengthened by the well kno^ii resemblances between 

 the modern Folypterus and the dipnoans in the early stages of develop- 

 ment. Nor should the differences in brain structure of these modern 

 forms outweigh the above mentioned resemblance, for there is no evidence 

 that the brains of the Devonian Crossopterygii and Dipnoi were any more 

 divergent from each other than were the other parts of the body. 



CROSSOPTERYGII 



The known Dipnoi being excluded from direct ancestry to the Tetra- 

 poda by reason of certain specializations of the skull and dentition, what 

 can be said of the claims of the Crossopterygii? 



Folypterus, and doubtless also its near ally Calamoichthys, which are 

 the only surviving crossopterygians, have become highly ichthyized in 

 brain characters and thus are far removed from both the dipnoan and 

 the amphibian types; but as stated above it may well be that the Devonian 



