154 NEW YORK STATE MUSEUM 



for the first time to identify positively the elements of the lower jaw in 

 Arthrodires, and to ascertain the nature of its articulation with the head 

 shield. That the shaft of bone which we have called the splenial is prop- 

 erly determined as such follows necessarily from the rule that in all higher 

 types of fishes and other vertebrates, the functional lower jaw is formed by 

 membrane bones which become ossified around Meckel's cartilage. In their 

 greatest development there may be several of these bones on either side, 

 amongst which the clentary is defined as that bone which is developed in 

 front — usually it is dentigerous — and the splenial that which is developed 

 further back on the inner side. The angulare, when present, extends for- 

 ward from the angle of the lower jaw to meet the other two, and in addition 

 a supra-angulare sometimes occurs behind the articulation of the lower jaw 

 with the quadrate. Meckel's cartilage itself never ossifies, but remnants of it 

 may persist in grooves or otherwise, as has already been observed. The man- 

 dibles of Dinomylostoma are of peculiar importance in that their constituent 

 parts are found to correspond precisely with those of Dipnoan fishes. 



The vomerine teeth of the present species are readily identifiable as 

 such, owing to their general similarity to the corresponding structures in 

 Dinichthys. They are prehensile to about the same degree as the sym- 

 physial beaks of the lower jaw, against which they closed. Their posterior 

 face is smooth and slightly concave, as if they had been in direct apposition 

 with the anterior pair of palato-pterygoid dental plates, but these latter are 

 missing in the only known example. The hinder pair, however, is admir- 

 ably preserved, and, its position being definitely fixed in the manner already 

 indicated, it is not difficult to restore the general outline, at least, of the 

 missing elements. Their external margin must have been parallel to that 

 of the functional surface opposed to them in the lower jaw ; and as they 

 were probably in contact along the median line, the inner mayjin was 

 rectilinear, as in Mylostoma variabile. An inspection of text ."igure 

 32 will facilitate an understanding of the manner in which upper and lower 

 dental elements closed together, it being borne in mind that the tips of the 

 vomerine and mandibular pair of plates were directly opposed. 



