190 IOWA GEOLOGICAL SUEVEY 



• the angle formed by the prongs of the opposing elements. The 



best extant illustration of the manner in which the lower and ' 

 upper dentition came together in front will be found in the 

 frontispiece of Dean's work on "Fishes, Living and Fossil."* 



The vomerine teeth are succeeded almost immediately behind 

 by the cleaver-like palato-pterygoid plates called by Dean 

 "orbito-gnathals", but popularly known as "shear-teeth", in 

 allusion to their mode of working 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 condi- 

 tion which is regarded as more primitive than that .with simply 

 sharpened, or beveled edges. Traces of an original denticula- 

 tion, 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 cor- 

 responds without question to the similarly placed process of 

 Ceratodont dental plates. Notwithstanding the large size and 

 evidently great efficiency of the shear-teeth, they do not appear 

 to have been rigidly attached to the headshield, but rather to 

 have been held in place by cartilage against the prominent in- 

 ferior ridge which extends forwards as far as the orbital region 

 from the postero-lateral angles, in a direction parallel with the 

 sides of the headshield. Precisely similar conditions are ob- 

 served in Neoeeratodus, where the ridges referred to serve as a 

 support for the upper dental plates, and relieve the strain in- 

 curred through the action of the jaws. It is interesting to note 

 that these ridges along the under side of the headshield in Din- 

 ichthys acquire greater solidity in proportion as the dental 

 plates become more masive and powerful. 



Not more than two elements are known to take part in the 

 formation of the mandible. These are the splenial, a long, 



*See also the more recent restoration by E. B. Branson, in the Ohio Naturalist, 

 1908, 8, pp. 365, 367. 



