22 E. B. BRANSON 



with small teeth belonging to the type which has been designated as Lophodus by 

 Romanowski, and the forms didymus and laevissimns, both named as species of 

 Helodus by Agassiz. 1 



During the progress of the present investigation a study of a por- 

 tion of the types of Psephodus crenulatus and P. obliquus, in addition 

 to the specimen already mentioned with the teeth in position, and 

 described in this paper as Psephodus legrandensis, has led to the fol- 

 lowing conclusions: The teeth called median mandibular by St. 

 John and Worthen 2 are posterior and correspond to the posterior 

 teeth of Cochliodus. The teeth called median maxillary by the 

 same authors are median, but not in the sense that they used the 

 term. They articulate behind with the large posterior plates and in 

 front with the small anterior teeth, and" do not articulate with helo- 

 doid teeth, as St. John and Worthen thought. In this genus there 

 is nothings that enables us to distinguish the maxillary from the 

 mandibular teeth. Teeth like those called mandibular by St. John 

 and Worthen have been found in place on the same jaw with those 

 called maxillary. (See Plate I, Fig. 2.) All of the teeth of Psephodus 

 that have been described belong in the categories hitherto known as 

 median mandibular and median maxillary teeth, and, as before said, 

 they occur on the same jaw. 



In previous publications the commonly expressed opinion has been 

 that helodoid teeth were present on the same jaw with the large 

 grinding plates of Psephodus, although very little evidence in sup- 

 port of this has been forthcoming. The only evidence that seems to 

 be of much weight in its support is that furnished by a specimen 

 described by Traquair in 1885. 3 In this specimen forty-four helo- 

 doid teeth are preserved on the same slab with two of the large 

 Psephodus teeth. It is no uncommon occurrence for teeth of different 

 species to be preserved in close association, and the mere fact of such 

 an association proves nothing, although, so long as no evidence to 

 the contrary was forthcoming, it might well suggest the possibility 

 that all the associated teeth belonged originally in the same jaw. 

 Traquair's interpretation of the original position of these teeth in 



1 Traquair, Geological Magazine, 1885, p. 343. 



2 Paleontology of Illinois, Vol. VII, Plate I, Figs. 1 and 2, pp. 66 and 67. 



3 Geological Magazine, 1885, pp. 337-44. 



