EMBRYOLOGY OF CRYPTOBRANCHUS 543 



tratioii of the teeth into tritoral chisters in the roof and floor of 

 the mouth is not in itself an unfavorable feature, for vomerine 

 teeth occur in the amphibia; but in all except certain very early 

 fossil forms (e.g., Uronemus) the fusion of these teeth into large 

 dental plates' with grinding ridges has gone too far to give rise 

 to the condition found in the amphibia. So far as the paleonto- 

 logical and anatomical evidence is concerned, the known facts tend 

 to exclude the dipnoi from the direct ancestry amphibia, of the 

 yet do not wholly preclude the possibility that future discov- 

 eries may supply us with more favorable material amongst early 

 fossil forms. In so far as the terrestrial adaptations of the dipnoi 

 resemble those of the amphibia, the case may be one of parallelism 

 or convergence; their more fundamental resemblances indicate 

 that they are not very far removed from a common ancestry. 



Turning to the crossopterygii we find more favorable anatom- 

 ical and paleontological grounds for comparison with the am- 

 phibia. The dermal elements forming the roof of the skull occur 

 in paired series; a large number of cranial bones may be definitely 

 homologized with those of the amphibia (see especially Baur '96; 

 Moodie '08 a; for materials for further comparison see Goodrich 

 '09, and Zittel '11). It is difficult to believe that identical rela- 

 tions in so many bones could have been independently evolved. 

 With regard to the fins, we find examples of a bifurcated type of 

 endoskeleton that makes a more favorable starting-point for a 

 tetrapod limb than the archipterygial type of the dipnoi; e.g., 

 see fig. 203 for the pectoral fin of Sauripterus, and Goodrich 

 '09, p. 275, fig. 244, for the pelvic fin of Eusthenopteron; the 

 pectoral fin of Eusthenopteron (Goodrich '09, p. 282, fig. 252) 

 is not quite so favorable. But both of these forms belong to the 

 Rhizodontidae, whose skull is not so favorable for comparison 

 with the- amphibia as the skull of some other crossopterygians ; 

 in no one form do we find all the conditions ideal for the deriva- 

 tion of the tetrapod type. The occurrence in Polypterus of two 

 kinds of ribs, both the ventral or pleural ribs characteristic of 

 the teleostomi and dipnoi, and the dorsal ribs characteristic of 

 elasmobranchs and tetrapods, is a point emphasized by Baur 



