234 



IOWA GEOLOGICAL SURVEY 



In view of the abundance of S. calvini and S. trisulcatus with- 

 in a limited area of the Iowa Upper Devonian, it is indeed 

 surprising that similar dental plates have not been brought to 

 light in other regions of the globe, either from the same, or 

 from earlier or later horizons. With the possible exception of 

 the genera Conchodus and Cheirodus as denned by M'Coy, the 

 latter occurring in the Lower Carboniferous,* it does not appear 



Fig. 34. 



Fig. 34. Dipterus valenciennesi Sedgw. & Murch. Lower Old Red Sandstone; Orkney. 

 Well preserved headshield showing cranial roofing plates and sensory canals, x 1-1. 

 Original in Yale Museum. 



*The so-called Conchodus plicatus, described by Dawson from the Coal Meas- 

 ures of Nova Scotia, is regarded by Smith Woodward as having been founded upon 

 an abraded dental plate of Ctenodus. 



