DEVONIC FISHES OF THE NEW YORK FORMATIONS 1 53 



Palaeomylus, especially P. greenei, and they are constructed more 

 nearly after the pattern of Dinichthys than of Mylostoma. This by no 

 means allows us to suppose, however, that the Mylostomid type is a deriva- 

 tive of Dinichthys, or of its immediate predecessor, 

 Coccosteus, inasmuch as the upper dentition in the 

 family we are considering is decidedly more primitive 

 than that of Coccosteans. The present species illus- 

 trates the manner in which the lower dental plates of 

 Dinichthys and its allies may have become modified 

 from the usual form of these organs amongst Dipnoan 

 fishes, by becoming turned upright in the jaws and 

 acquiring a trenchant functional margin through gradual teecheri, as seen from in 



^ ° _ ^^ _ . front and slightly to one side. 



compression. One can readily conceive the possibility Nat. size 



of the palatopterygoid plates undergoing corr-esponding modifications, 



which resulted in the development of shear teeth. 



In the mandibles of the species under discussion, the symphysial beak 

 is rather obtuse, and but slightly prehensile, being scarcely elevated above 

 the broad, flat, deeply excavated functional surface. The latter displays a 

 single inconspicuous eminence or tubercle close to the external margin, 

 situated about midway the length of the oral margin ; and at a distance of 

 about 8 cm behind the anterior beak is a larger tubercle, rather elongate, 

 and placed externally like the first. This posterior prominence fits accu- 

 rately against the single large rounded boss of the opposing upper dental 

 plate, thus determining the orientation of the latter with absolute certainty, 

 and affording a trustworthy clue to the position of the corresponding- 

 element in M. var labile. 



The splenial is developed as a long slender shaft of bone, resembling 

 that of Dinichthys, but relatively deeper. Both of these elements, right and 

 left, are preserved in natural association with the articular cartilage. This 

 has become more or less compressed through fossilization, but remains 

 attached to the outer face of the splenial near Its posterior extremity. 

 Owing to the favorable preservation of the specimen, we are now enabled 



