DEVONIAN FISHES OF IOWA 195 



A primitive species seldom exceeding and usually somewhat 

 smaller in size than D. intermedins, distinguished from it and 

 its contemporaries in the Ohio Upper Devonian by its fine tuber- 

 culation, undulating suture lines, and decidedly Coccosteus-like 

 aspect. Lower dental plates with a simple trenchant margin, ter- 

 minating behind in an abrupt downward declivity bearing a se- 

 ries of few, regularly spaced rudimentary denticles. Palato- 

 pterygoid dental plates ("shear-teeth") with convex functional 

 margin, simply trenchant, and not denticulated along the poster- 

 ior border so far as known. Vomerine teeth resembling those of 

 D. intermedins. Visceral surface of occipital region without 

 prominent ridges, and posterior pit on under side of the median 

 occipital scarcely divided. Pineal plate apparently in contact 

 with the centrals, and with inconspicuous foramen. 



Of this species, which appears to have been rather abundant 

 and widely distributed in the Middle Devonian of this country, 

 nearly the entire dermal armor is known, and the whole of the 

 dentition. Amongst the primitive characteristics of the form in 

 question may be reckoned its fine, Coccosteus-like tuberculation, 

 sinuous suture lines, remnants of an original denticulation along 

 the sloping posterior margin of the lower dental plates, and the 

 comparatively slight development in the latter of a toothlike pro- 

 jection at no great distance behind the symphysial beaks. At 

 the same time, however, it must be acknowledged that this 

 species marks a considerable advance over typical Coccosteus- 

 like conditions, inasmuch as the functional margin of the dental 

 plates is no longer serrated, the dorsomedian plate has de- 

 veloped a strong inferior carina and posterior process, becom- 

 ing also emarginate in front, and a clavicular occurs of the usual 

 Dinichthyid type. 



The occurrence of D. pustulosus in the New Albany Black 

 Shale of Kentucky, a horizon corresponding approximately to 

 the Genesee of New York State, favors the supposition that it 

 was the immediate progenitor of forms like D. intermedins and 

 D. terrelli of the Cleveland shale, which have retained a similar 

 form of dentition. According to this view, D. herzeri and other 

 species in which the functional margin of the dental plates is 

 denticulated, form a separate series, descended along collateral 



