Eastman — Dipnoan Affinities of Arthrodires. 135 



topterus, even imagining the latter to be a lineal descendant 

 of " Placoderms." Now the trenchant dental plates of Protop- 

 terus and Lejpidosiren are clearly hut a variant of the Cerato- 

 clont type, and we have to take only a step further to see that 

 the Dinichthyid dentition has been similarly derived. No dif- 

 ficulty is offered by the so-called " premaxillary " teeth of Din- 

 ichthys, which are the precise equivalents of the vomerine 

 pair in modern Dipnoans. As for the characteristic crushing 

 plates in upper and lower jaws of Ceratodonts, these occur nor- 

 mally 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* 

 which may be taken to represent an incipient stage of meta- 

 morphosis. 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 

 projection in Dinichthyid mandibles. 



As for the so-called " maxillary " or " shear-tooth " of Din- 

 ichthys, this corresponds plainly to the triturating upper (pal- 

 ato-pterygoid) dental plate 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.f In Arthrodires, as in other Dipnoans 

 and higher forms, the functional lower jaw is formed by mem- 

 brane plates which have ossified around the Meekelian carti- 

 lage. Distinct angular and articular elements appear to be 

 wanting in Dinichthys, but the splenial is strongly developed, 

 supporting the dental plate properly speaking in front, and 

 being articulated posteriorly with the cranium by cartilage, as 

 in Neoceratodus. But one interpretation can be given of the 

 conspicuous groove which extends forward along the inferior 

 border of the splenial, passing underneath and to the inner 

 side of the dental plate proper, and terminating at the sym- 

 physis. In it were lodged remnants of the Meekelian cartilage, 

 precisely as in the living Protopterus.% Intermandibular 

 teeth have not been definitely proved to occur, although their 

 presence would be in strict accord with embryological evi- 

 dence, and the appearance of certain specimens has created a 



* Teller, F., Ueber den Schadel ernes f ossilen Dipnoers, Abhandl. k. k. 

 Keichsanstalt Wien, vol. xy (1891), pt. 3, pi. iv. 



fThis process is well shown in Plate la, fig. 4, of Miall's " Monograph of 

 the Sirenoid and Crossopterygian Ganoids." Palteont. Soc, 1878. 



:fWiedersheim, E., Morpholog. Studien, Heft 1, p. 55, pi. ii, figs. 3, 8. 

 Fiirbringer, K., op. cit., p. 481, pi. xxxix, fig. 28. 



