366 PKOP. T. W. BRIDGE ON THE MOBPHOLOGT OF THE SKULL 



genera are much the same in nature and extent as those which distinguish the skull 

 of first- and second-year Frogs. 



It cannot, however, be too strongly emphasized that in any comparison of the skulls 

 of Protopterus and Lepidosiren it is important that due regard should be paid to the 

 size and age of the specimens upon which such comparison is based. According to 

 Giinther [12, p. 357] Protopterus sometimes attains a length of six feet, whereas most 

 of the specimens which have reached Europe, and furnished the needful material for 

 the hitherto-published papers on the structure of the skull, seem to have been com- 

 paratively small and relatively young examples in which it is improbable that the skull 

 had attained its final and complete development. Hence it is by no means improbable 

 that, if comparison were made with larger and older specimens of Protopterus, some at 

 least of the cranial modifications in Lepidosiren, which have been mentioned as 

 distinctive of the genus, would either disappear or become so far softened down as to 

 lose much of whatever importance they now seem to possess. That this is probable is 

 evident from the fact that certain of the more obvious differences in the cranial structure 

 of the two genera are precisely those which might easily be bridged over by older 

 specimens of Protoj'teriis. On this point it is only necessary to refer to the greater 

 development of the fronto-parietal, the further retrogression of the chondrocranium, 

 and the greater backward rotation of the suspensorium in Lepidosiren. 



IV. The Structure of the Skull in Fossil Dipnoi. 



When the range of comparison is so far extended as to include the extinct Dipnoid 

 families, the Dipteridse, Phaneropleuridee, and Ctenodontidse^, it is obvious that, while 

 the fossil types exhibit a general agreement with existing Dipnoi in the main outlines 

 of their cranial structure, they differ from the latter, as well as from one another, in 

 several striking and significant features. Broadly speaking, it may be affirmed that 

 all known fossil Dipnoi agree with their living representatives and differ from all 

 other Fishes in possessing the following combination of cranial characters : — 



(A.) Complete and typical autostylism. 



(B.) The presence of characteristic triturating palatal teeth, supported by palato- 

 pterygoid bones, symphysially united beneath the ethmoidal region of the 

 skull, and probably associated with the absence of premaxillse and maxillae 

 in the upper jaw. 



' I have here followed the classification of the fossil Dipnoi as given by Smith Woodward [45]. Traquair 

 [39] has suggested the separation of f/roneimis from the Phaneropleuridae as the type of a distinct family 

 [Uronemidce], and the union of the remaining genus Phunerojileuroii with the Diptcridse and Ctenodontidse in one 

 comprehensive family of Ctenodontidae. 



