138 Eastman — Dvpnoa/n Affinities of Arthrodires. 



On referring- to the diagram in fig. 1, it will be seen that 

 the vomerine teeth in Mylostoma are succeeded behind by 

 two pairs of palato-pterygoid plates, instead of one only, as in 

 Ceratodonts. Embryology teaches, however, that the discrep- 

 ancy is apparent, not real ; for as shown by Semon,* the den- 

 tal plates of Neoceratodus arise through concresence of conical 

 denticles, which are at first disposed so as to form two pairs of 

 palato-pterygoid plates, arranged as in Mylostoma, these after- 

 wards fusing into one. Clearly, therefore, the upper dentition 

 of Neoceratodus passes through an early Mylostoma-st&ge. 

 Further, it will be noted that the peculiar posterior contour of 

 the hindermost pair of plates in Mylostoma (the same holds 

 true also for Dinomylostoma) is conformable to, and therefore 

 to be associated with, the usual pattern of palato-pterygoid 

 cartilage found in all Dipnoans. Amongst Ctenodipterines 

 this element is ossified, and passes under the name of '' upper 

 dentigerous bone " ; the fact that it is unossified in Arthro- 

 dires agrees with other evidence pointing to their lesser spe- 

 cialization. 



The mandibles of Mylostoma betray unmistakable indica- 

 tions of a Ceratodont origin, for the functional dental plate is 

 even more sharply demarcated from the supporting splenial 

 than in Dinichthys or Dinomylostoma. It even bears prom- 

 inent ridges radiating from the inner margin, which may 

 possibly be a survival of primitive Dipnoan conditions. The 

 presence of intermaudibular teeth would be in complete har- 

 mony with embryological 'evidence, and as a matter of fact, 

 certain detached teeth have been somewhat doubtfully inter- 

 preted as such by Newberry. f The pair figured by him, how- 

 ever, have every appearance of being vomerine ; interpreting 

 them as such, we must confess ignorance of a pair opposed to 

 them in the lower jaw. Amongst Ctenodipterines, Synthe- 

 todus is the only genus in which symphysial teeth are perma- 

 nently retained, thus paralleling an evanescent stage of Neo- 

 ceratodus. 



Cranial Characters of Arthrodires and Neoceratodus com- 

 pared. — The most notable peculiarity of the skull in Neocera- 

 todus^ as compared with Ctenodipterines, is its retention 

 throughout life of a completely closed and almost entirely 

 unossified chondrocranium. The contrast presented by Dij?- 

 terus in this respect is very striking, Traquair's claim being 

 amply supported that this genus attained a higher grade of 



*Semon, R. , Die Zahnentwickelung des Ceratodus forsteri. Zool. For- 

 schiingsreisen in Australien. Jena Denkschr. , vol. iv (1901), pp. 115-133. 



f Newberry, J. S., Paleozoic Fishes of North America, Monogr. U. S. 

 Geol. Surv., vol. xvi (1889), p. 165, pi. xvi, fig. 4. 



