EASTMAN: STRUCTURE AND RELATIONS OF MYLOSTOMA. 7 



between Artlirodires and Ceratodonts. First and most important of all, 

 the characters furnished by the dentition may be considei-ed ; and we 

 shall endeavor to show that Dinichthys and Mylostoma represent the 

 same modifications in the ancient fauna as are displayed by Protopterus 

 and Neoceratodus in the recent, so far as dental characters are concerned. 



Dentition of Dinichtliys and Neoceratodus compared. — It is admitted 

 by all writers that Neoceratodus, as compared with the two other sur- 

 viving genera of lung-fishes, represents a relatively early larval stage of 

 development ; nor is it questioned by any one that the trenchant dental 

 plates of Protopterus and Lepidosiren are not mere variants of the Cera- 

 todont type. This much being already clear, it is but a short step further 

 to see that the dentition of Coccosteus and Diniclithys has been similarly 

 derived. Certainly no difficulty is offered by the so-called "premax- 

 illary " teeth of Dinichthys, which are the precise equivalent of the 

 vomerine pair in modern Dipnoans, as was long ago pointed out by Dr. 

 Theodore Gill.^ As for the characteristic tritoral plates in upper and 

 lower jaws of Ceratodonts, these occur normally in Mylostoma, but in 

 Dinichthys have become rotated so as to stand upright in the jaws, 

 their outer denticulated margins functioning against one another like 

 the blades of a pair of shears. An inkling as to how this variation was 

 brought about is afforded by the Triassic Ceratodus sturii Teller, '^ which 

 may be taken to represent an incipient stage of metamorphosis. The 

 dental plates of this form are seen to be turned more or less on edge, 

 the corrugations interlocking in opposite jaws when the mouth is closed, 

 and a rudimentary beak being developed in front which recalls the well- 

 known tooth-like projection in Dinichthyid mandibles. 



As for the so-called " maxillary " or " shear-tooth " of Dinichthys, this 

 corresponds plainly to the triturating upper (palato-pterygoid) dental 

 plates of Ceratodonts, turned rather more upright than in C. sturii; 

 and its anterior process or "shoulder" is represented by the forwardly 

 placed ascending process of modern forms. 



The functional lower jaw of Arthrodires agrees with that of other 

 Dipnoans in that the mandibular dental plate is supported solely by the 

 splenial, and no true dentary element is present. The Ctenodipterine 

 mandible, as compared with that of other Dipneusti, is the most compli- 

 cated, being composed of a greater number of pieces, more extensively 

 ossified, and covered externally with a ganoine investment. Consider- 



1 Kept. Geol. Surv. Oliio. Paleont., 1875, 2, p. 7. 



2 Teller, F. Ueber deu Schadel eines fossilen Dipnoers. Abhandl. k. k. Reich- 

 fianst., Wien, 1891, 15, Plate iv. 



