228 bulletin: museum of comparative zoology. 



" premaxillary " teeth in Diuichthys, and the other of the so-called 

 " maxillary " or " shear-teeth." The hypothesis of an additional paired 

 element in advance of these two, itself representing the " premaxillary " 

 in Dinichthys, is not considered in the case of Mylostoma, and rejected 

 in the case of Dinomylostoma. The tritoral plates are arranged in such 

 manner that the mandibles cannot close against them directly so as 

 to produce the observed marks of contact without operating by rotary 

 movements, and without being capable of approximation and separation 

 at their anterior extremities, — conditions which are unparalleled among 

 Chordates. All the jaw-parts are regarded as of purely dermal origin, 

 and therefore non-homologous with those of ordinary fishes. Their 

 alleged structural diffei-ences, and assumed functional differences, make 

 it necessary to exclude Arthrodires from fishes proper. 



The consequences depending upon the newer reconstruction of the 

 same materials are that all known Dinichthyids and at least one Mylo- 

 stomid have a similar form of " premaxillary," which is the exact 

 homologue of the vomerine teeth in Dipnoans, and that the succeeding 

 pair or pairs (when two are present) of trenchant or crushing plates 

 are homologous with the palato-pterygoid dental plates of typical 

 Dipneusti. The jaws operate in the usual manner, are of the normal 

 gill-arch type, and exhibit precisely the same conformation as those 

 belonging to autostylic fishes. The combined evidence of the majority 

 of characters of Arthrodires proves that they are specialized Dipnoans. 



