g6 NEW YORK STATE MUSEUM 



point only need be developed a little more fully, perhaps, and that is the 

 fact that Dinichthys and Mylostoma are related to each other in the same 

 way with respect to their dentition as are Protopterus and Neoceratodus. 

 This will appear from the following considerations. 



Dentition of Dinichthys and Neoceratodus compared. It is recognized 

 by all students that Neoceratodus, in comparison with other surviving lung 

 fishes, represents a relatively early larval stage of development ; nor does 

 any one question that the trenchant dental plates of Protopterus and Lepid- 

 osiren are other than a mere variant of the Ceratodont type. This much 

 being granted, it is but a short step further to see that the dentition of 

 Dinichthyids has been similarly derived. Certainly no difficulty is offered 

 by the so called " premaxillary " teeth of Dinichthys, which are the precise 

 equivalent of the vomerine pair in modern Dipnoans. As for the character- 

 istic crushing plates in the upper jaw of Ceratodonts, these occur normally 

 in Mylostoma, but in Dinichthys become rotated so as to stand more or less 

 vertically, their outer denticulated margins functioning against one another 

 in opposite jaws like the blades of a pair of shears. An inkling as to how 

 this modification was brought about is offered by the Triassic Ceratodus 

 s t u r i i, which may be taken to represent an incipient stage of metamor- 

 phosis. The dental plates of this form are observed to be turned consid- 

 erably on edge, their marginal corrugations interlocking in opposite jaws 

 when the mouth is closed, and a rudimentary beak being developed in front 

 which recalls the well known forward projection in Dinichthyid mandibles. 



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

 corresponds clearly to the triturating upper (palato-pterygoid) dental plate 

 of Ceratodonts, turned rather more upright than in C. s t u r i i, and its 

 anterior process or " shoulder " is represented by the forwardly placed 

 ascending process of modern forms. In Arthrodires, as in other Dipnoans 

 and higher forms, the functional lower jaw is formed by membrane plates 

 which have ossified around the Meckelian cartilage. Distinct angular and 

 articular elements appear to be wanting in Dinichthys,' but the splenial is 



'Dean, B. Fishes, Living and Fossil. N. Y. 1895. p. 133. 



