144 IOWA GEOLOGICAL SURVEY 



The spines assigned to this species are scarcely distinguish- 

 able from those of Acantholepis, such differences as are to be 

 noted in form, segmented structure and tuberculate ornamenta- 

 tion being only of minor importance. These Hamilton spines 

 are evidently constructed upon the same peculiar pattern as 

 those that occur typically in the Onondaga of New York and 

 Ohio, described by Newberry as Acantholepis. Ignorant as we 

 are, however, of their precise relations, it seems advisable to 

 maintain a provisional distinction between the two genera and 

 species, until such time at least as an identity is established be- 

 tween slightly dissimilar spines occurring in different divisions 

 of the Middle Devonian. The species which is thus convention- 

 ally distinguished under the above name is not known to occur 

 elsewhere than in the Hamilton limestone of Milwaukee, Wis- 

 consin. Palceomylus greenei appears also to be limited to the 

 same horizon and locality, and other species of Pala?omylus, P. 

 crassus and P. f rang ens, accompany the spines of Acantholepis 

 in the Ohio Onondaga. The coincidence suggests with some 

 plausibility that Palaeomylus may have been armed with seg- 

 mented spines of this character. 



Formation and locality. Hamilton limestone; Milwaukee, 

 Wisconsin. 



Genus acanthaspis Newberry. 



A purely provisional classificatory title which may for the 

 present be conveniently retained for certain spiniferous plates 

 occurring in the Lower and Middle Devonian of this country and 

 Europe, but known as yet only in the detached or dissociated 

 condition. There are, however, excellent reasons for supposing 

 these structures to be dermal ossifications belonging either to 

 Rhynchodus or some closely related genus, although heretofore 

 they have been variously classed with Oephalaspids, Bothrio- 

 lepids and Arthrodires, especially such forms among the last- 

 named as Phlyctaenaspis. 



The chief reason for supposing the spiniferous bodies de- 

 scribed under this name and the dental plates of Rhynchodus to 

 belong to one and the same sort of creature depends upon the 

 recognition of homologies between the plates known as Acan- 



