370 PBOF. T. W. BRIDGE ON THE MORPHOLOaT OF THE SKULL 



remaining genera—Scatmenacia, Phaneropleuron, Uronemus, and Ctenodiis, in the 

 order mentioned— representing successive grades of modification in the same general 

 direction, and culminating in the living genera Ceratodus, Protopteriis, and 

 Lepidosiren as the final terms of an evolutionary series. It is not suggested that the 

 sequence of the genera represents descent in a direct line, but rather that the genera 

 Hre to be regarded as lateral off'shoots of the main line of descent, while at the same 

 time illustrating the general direction of the evolution of the group. Commencing 

 with Dipterus valenciennesi, the principal modifications in cranial structure which the 

 difierent genera exhibit in ascending order are {a) reduction in the number of the 

 cranial dermal bones ; {b) the gradual loss of their investment of ganoin ; (c) the 

 suppression of the jugal plates ; and {d) reduction in the size and importance of the 

 opercular elements ^. 



Viewed from the standpoint of cranial structure alone, there are grave difficulties 

 which militate against, the acceptance of at least one, and that perhaps one of the 

 most important, of Dollo's conclusions. So far from Dipterus being the most primitive 

 of known Dipnoi, it is obvious, as Traquair [37] has pointed out, that in some of its 

 cranial features this genus has attained a higher grade of specialization than any 

 existing Dipnoid. In proof of this it is only necessary to refer to the more extensive 

 ossification of the chondrocranium, the presence of a quadrate bone as the functional 

 suspensorium, and the existence of an articular element in the lower jaw, all of which 

 are features without parallel in any of the living genera. If we admit the accuracy 

 of Dolln's view, that Dipterus represents the most primitive type of the Dipnoid stock, 

 it will be necessary to assume the possibility of an ossified skull so far degenerating 

 as to lose almost all trace of endochondrial ossification, and secondarily revert to 

 the condition of a skull so completely cartilaginous, and so primitive in other respects, 

 as that exhibited by the living Ceratodus. So far as I am aware, there is no evidence 

 to justify belief in such a possibility. 



As to the relations of the fossil Dipnoi to one another and to their living allies, it is 

 difficult to arrive at any satisfactory conclusion from the facts of cranial structure 

 alone, and the diflficulty is increased by the want of definite and precise information on 

 many important points in connection with the fossil Dipnoi, more particularly as to 

 the condition of the chondrocranial portion of the skull. On the evidence at present 

 available it seems probable that Ceratodus is the most primitive of known Dipnoi, and 

 that Protopterus and Lepidosiren are the specialized and direct descendants of some 

 Ceratodns-\We ancestor. Of the fossil Dipnoi, Ctenodus certainly, and possibly 

 also Phaneropleuron, approach more closely to Ceratodus than do Dipterus and its 

 allies. The last-mentioned Dipnoi, on the contrary, seem to represent a divergent 

 and terminal branch of the Dipnoid stem, and to include the most highly-specialized 

 examples of the group. 



* It must be mentioned that Dollo's comparison is not restricted to the skull, but includes also the mesial 

 and paired fins, the scales and other structures of the various fossil and living genera. 



