DEVONIC FISHES OF THE NEW YORK FORMATIONS 1 23 



shorter of which is inwardly placed and is scarcely separated from its fellow 

 of the opposite side. 



At least one Dinichthyid individual is known in which the vomerine 

 pair is solidly fused with the supporting bones of the head shield The 

 specimen referred to forms part of the collection of Ohio fishes brought 

 together by Dr William Clark and eventually acquired by the British 

 Museum of Natural History. It is catalogued under the number P. 9490, 

 and in the opinion of Dr A. S. Woodward, keeper of the Geological Depart- 

 ment, properly belongs to the genus we are now considering. Another 

 interesting specimen having the upper and lower dental elements preserved 

 in situ, is preserved in the same collection under the catalogue number 

 P. 9340. By means of this and similarly preserved material, and also by 

 examining the marks of wear in upper and lower dental plates, one may 

 readily determine the manner in which the dental parts came together when 

 the mouth was closed. The vomerine teeth are seen to have protruded just 

 a trifle in advance of the mandibular beaks, which closed within the angle 

 formed by the prongs of the opposing elements. A few plates of Dinich- 

 thyid armor are also known, which have been bitten by creatures of their 

 own kind in such wise as to leave the impress of teeth ; and these markings 

 confirm in an unexpected way the observations we have just made concern- 

 ing the arrangement of jaw parts, besides affording indications of the dis- 

 tance apart of the symphysial beaks. The best illustration that has been 

 offered of the manner in which the upper and lower dentition came together 

 in front will be found in the frontispiece of Dr Bashford Dean's work on 

 Fishes, Living and Fossil. Some novel suggestions concerning the inter- 

 play of the jaw parts, which are assumed to have been freely mobile, are 

 contained in Dr L. Hussakofs Studies on the Arthrodira, published in 

 volume IX of the Memoirs of the American Museum of Natural History, 

 1906. 



The vomerine teeth are succeeded almost immediately behind by the 

 cleaverlike palato-pterygoid plates, called by Dean " orbitc-gnathals," but 

 commonly known as "shear teeth," in allusion to their mode of working 



