DEVONIAN FISHES OF IOWA 229 



pies of the same species have been figured but not described in 

 Volume VII of the Survey Reports (1896), pi. iv. figs. 27-32, 

 and after pointing out their resemblance to the forms described 

 by M'Coy and Pander it was remarked that these teeth "are 

 seen to pass by gradual transitions into true Dipterid forms." 

 (torn, cit., p. 113.) 



A comparison of the dental structures shown in Plates VIII 

 and IX of the present work may suggest the query whether or 

 not these may be the upper and lower dental plates of one 

 and the same species, the paired lateral elements being fused in 

 the upper pavement to form a single compound plate, but remain- 

 ing discrete and separated from each other in the lower jaw. 

 This interpretation was at one time adopted by the writer as 

 perhaps the most plausible working hypothesis, and at the same 

 time the simplest. The inclusion of all these smooth dental 

 plates under a single species seemed to be further sanctioned 

 by the fact that no differences in microscopic structure could be 

 observed upon comparison of a large number of thin sections. 



Professor Calvin, however, who possesses a magnificent col- 

 lection of State Quarry fish-teeth, remained consistently op- 

 posed to the theoretical association of parts just indicated, and 

 preferred to regard the originals of Plates VIII and IX as be- 

 longing to two distinct categories which required generic sep- 

 aration. A recent critical review of the arguments in favor of 

 a separation has convinced the present writer of the entire rea- 

 sonableness of Professor Calvin's view, and it is accordingly 

 adopted as the one most likely to be correct. There are in fact, 

 two difficulties which effectually prevent us from identifying the 

 originals of Plate VIII (apart from Figs. 16, 20, 29 and 34) as 

 the mandibular dental plates of Synthetodus, and these may be 

 stated as follows : 



(1) Although the majority of Dipnoan teeth shown in Plates 

 VIII and IX have a convex functional surface, and are thus, 

 after analogy with Dipterus, theoretically assignable to the 

 lower jaw, yet there are numerous other teeth agreeing with 

 the types represented in each plate referable to each of the cate- 

 gories here represented, yet differing from the originals of these 

 plates in that their functional surface is flattened or even slight- 



