230 IOWA GEOLOGICAL SURVEY 



ly concave. Assuming, therefore, that the convex teeth of each 

 type represent the mandibular dental plates, corresponding to 

 these are two other types of flat or concave teeth, which must 

 be supposed to represent the upper or palatal dentition. 



(2) In the Synthetodus type of Dipnoan dentition radiating 

 plications of the functional surface are absent, nor are any ves- 

 tiges to be observed of rows of tubercles along the ectal margin. 

 On the other hand in the Conchodus type of dentition plications 

 of this nature occur, although often indistinct, and both upper and 

 lower dental plates frequently display obsolescent rows of tuber- 

 cles. Owing to these constant differences it appears more pru- 

 dent to maintain the distinctness of the two types, at the same 

 time admitting the probability that they are serially related, 

 one having been derived as a modification of the other. None of 

 the examples of Conchodus variabilis chosen for illustration in 

 Plate VIII show the external plications very distinctly along the 

 external margin, but a few still preserve traces of the obsoles- 

 cent rows of tubercles. For instance, the terminal projection to 

 be seen in the originals of Figures 4 (inverted), 33, 15, 26, 28, 

 33, etc., is plainly a remnant of a tuberculated ridge, and other 

 specimens might have been selected for displaying this character 

 more perfectly. 



Conchodus variabilis, sp. nov. 

 (Plate VIII, with the exception of Figs. 16, 20, 29, 34) 



Founded upon Dipterus-like dental plates of small or moderate 

 size, with smooth or nearly smooth functional surface and only 

 vestigial remnants of tuberculated ridges along the outer mar- 

 gin; the number of these plications or short rows of indistinct 

 tubercles rarely exceeds one or two. Fimctional surface of man- 

 dibular dental plates strongly convex, that of palatal dental 

 plates flat or slightly concave. 



Hundreds of specimens of smooth, elongate-ovoidal, more or 

 less strongly convex dental plates agreeing in form with the 

 majority of those shown in Plate VIII, but displaying, never- 

 theless, a wide range of variation, have been collected from the 

 "fish-bed" stratum at the famous State Quarry locality near 

 North Liberty, Iowa. At one time thought to represent the lower 



