124 NEW YORK STATE MUSEUM 



against the trenchant margin of the lower dental plates like the blade of a 

 pair of shears. The manner of their operation remains the same even in 

 those species where the opposing margins are denticulated, a condition 

 which is regarded as more primitive than that with simply sharpened, or 

 beveled edges. Traces of an original denticulation, which once extended 

 along the entire functional margin, are often observed in the form of 

 tubercles, or denticles, whose position is confined in specialized species to 

 the extreme posterior margin of the tooth. It is to be noted that this 

 posterior margin is usually narrower and more rounded than the anterior. 

 Slightly in advance of the middle portion of the tooth, along its superior 

 margin, there is given off from this upper margin a well marked, inwardly 

 curved ascending process or " shoulder," which corresponds without question 

 to the similarly placed process of Ceratodont dental plates. Notwithstand- 

 ing the large size and evidently great efficiency of the shear teeth, they do 

 not appear to have been rigidly attached to the head shield, but rather to 

 have been held in place by cartilage against the prominent inferior ridge 

 which extends forwards as far as the orbital region from the posterolateral 

 angles, in a direction parallel with the sides of the head shield. Precisely 

 similar conditions are observed in Neoceratodus, where the ridges referred 

 to serve as a support for the upper dental plates, and relieve the strain 

 imposed by the action of the jaws in mastication. It is interesting to note 

 that these ridges along the underside of the head shield in Dinichthys 

 acquire greater solidity in proportion as the dental plates become more 

 massive and powerful. 



Finally, attention may be directed to the lower dental plates strictly 

 speaking, the . term of " dental plate " being properly limited to the func- 

 tional, anterior portion of the mandibular arch, with which element it is 

 fused. The relations between the lower dental plate and the supporting 

 ossification, or splenial, and the manner in which it is supposed to have 

 become rotated so as to stand upright in the jaws, are matters that have 

 been sufficiently explained in the general account of Arthrodires [c/. siipra^ 

 p. 96]. In almost all species of Dinichthys, the anterior extremity of 



