EASTMAN: STRUCTURE AND RELATIONS OF MYLOSTO.MA. 9 



microscopic structure/ demonstrate in clearest possible manner that 

 definite homologies exist. Not only can there be no question as to real 

 homology, but it is further evident that one general type of mandible is 

 common to all Dipnoans, only amongst Arthrodires this type is resolved 

 to its simplest terms. So far as the present writer is aware, no adequate 

 cause has been shown for supposing that the jaws of Arthrodires were 

 capable of anomalous movements, and the notion that the mandibular 

 rami were not rigidly united with each other at the symphysis may be 

 resarded as slender as the seven lean kine. That the vomerine teeth, at 

 least, were immovably attached to the headshield is proved by tlieir 

 occasional fusion with it in Dinichthys, as in a specimen belonging to 

 the British Museum (Cat. No. P 9490), and presumably also in the 

 complete skull described by Newberry ^ " with the great premaxillary 

 teeth in place," immediately behind which were the "maxillaries." 

 There can be no question that the upper pavement dentition of Mylos- 

 tomids was absolutely fixed. Under such conditions it is inconceivable 

 that the mandibular rami were capable of torsion, and of separation and 

 approximation from each other at their anterior extremities. But it has 

 been argued that such movements are implied by the presence of sym- 

 physial denticles in forms like Coccosteus and Dij)lognathus. The logic 

 involved does not appear to be particularly convincing. In the first 

 place it is uncertain whether these denticles were really functional. And 

 in the second place, even if they were, their origin is best explained as a 

 reminiscence of primitive conditions, such as are to be inferred from the 

 ontogeny of Neoceratodus.^ 



Dentition of Mylostoma and Neocei'atodus compared. — The large tri- 

 toral upper dental plates of ]Mylostoma present such an obvious similarity 

 to the well-known crushing plates of typical Dipnoans that, supposing 



1 It has been shown by Claypole in the Proc. Amer. Micros. Soc, 1894, 15, 

 p. 189-191, tliat the functional margin of a jaw element in Dinichthys differs from 

 the remaining portion (splenial) only in its denser structure. Identical conditions 

 exist amongst fossil and recent Dipnoans. Thus Giintlier, in his description of 

 Neoceratodus, remarks that tlie substance of the splenial " passes so gradually into 

 that of the tooth that it is only by the difference in shade of color tliat the boundary 

 line between osseous base and dentinal crown is indicated. ... In our specimens 

 the structure of the bony base of the tooth differs in nothing from that of the 

 remainder of tlie dentary bone \i. e., splenial] : there is the same spongeous struc- 

 ture, the same proportion of bone-corpuscles, etc." — Phil. Trans., 1871, 161, p. 519. 



2 Newberry, J. S. Monogr. U. S. Geol. Surv., 1889, 16, p. 146. 



^ Semon, R. Die Zahnentwickelung des Ceratodus forsteri. Sitz. Gesell. Morph. 

 Phys. IMiinchen, 1899, 15, p. 75-85. Also Zool. Forschungsreisen in Australien, 

 Jena Denkschr., 1901, 4, p. 115-133. 



