64 PALEONTOLOGY OF ILLINOIS. 



First, teeth presenting a trapezoidal outline, the crown traversed 

 longitudinally by a low, obtuse-crested ridge, culminating posterior 

 of the median line, the inner margin somewhat angularly rounded, 

 outer extremity always truncate with channeled, crenulate articular 

 surface, the lateral borders undulated as though for coadaptation with 

 the lateral articular extremities of narrow serial teeth. 



Second, teeth generally more obliquely trapezoidal in outline, in 

 which the coronal ridge is relatively inconspicuous, and the inner 

 margin gently arched, from which the lateral borders less rapidly 

 converge towards the outer extremity, their articular surfaces also 

 undulated. Coronal surface minutely punctate. The above forms 

 are provisionally referred to opposed positions on the lower and upper 

 jaws respectively, chiefly on account of the somewhat stronger re- 

 semblance of the first mentioned form with the contour of the man- 

 dibular teeth of typical Cochliodonts. With the above remains are 

 associated narrow or transversely elongate teeth, whose crenulated 

 edges and coronal contour clearly point to their intimate kinship 

 with the preceding forms. The latter include the European form 

 originally described under the term Helodus planus, Agass., which 

 was by Capt. Jones regarded as belonging to Psephodus magnus. 

 There are other similar forms more or less numerously represented 

 in the collections, which may also prove to have been associated 

 with those above noticed. 



In reference to the dentition of Psephodus our information is still 

 meagre, although the identity of a variety of seemingly widely di- 

 verse forms may be asserted with a degree of confidence. While 

 certain conspicuous forms belonging to the genus may be compared 

 with the dental elements met with in Cochliodus, there was unques- 

 tionably a marked contrast in the character of the combined dental 

 armament of the jaws of those genera. Indeed, the resemblances 

 between them are but little more intimate than obtains in the in- 

 stance of Cestracion and Cochliodus. While the median portion of 

 the rami of the jaws of Psephodus was enveloped by a moderately 

 contorted dental plate, constituting its chief point of resemblance 

 with Cochliodus, this plate was flanked on either side by series of 

 teeth disposed in rows from within outward similar to the occur- 

 rence of the teeth upon the jaws of Cestration. Therefore, the solid 

 triturating plates of Psephodus are not strictly homologous with the 

 large posterior teeth of Cochliodus, but they are more properly des- 

 ignated as "median" teeth of the rami of the jaws. 



