206 



IOWA GEOLOGICAL SURVEY 



plates, none in association with the dentition or headshield, hence 

 their precise systematic position is doubtful. Coarsely tuber- 

 culated plates, suggestive of Newberry's type of Aspidiclithys 

 clavatus, are reported by Professor "Williams from the New 

 York Portage.* Similar fragments occur sparsely in the New 



Fig. 32. 



Fig. 32. Dorso-median plate of a small unknown Arthrodire surviving as late as the 

 dawn of the Carboniferous. From nodule layer at base of the Waverly in Boyle county, 

 Kentucky. Original in Museum Comparative Zoology, x 1-1. 



Albany (= Genesee) Black Shale near Louisville, Kentucky, and 

 Jeffersonville, Indiana. Others of equal thickness, but some- 

 what less coarsely tuberculated, are among the rarer fossils of 

 the State Quarry beds (Upper Devonian) in Johnson county, 

 Iowa. 



As implied by the generic name, the ornament of Holonema 

 consists of threadlike, radiating ridges. Two species are known, 

 the type (H. rugosum Claypole) occurring in the Chemung of 

 New York and Pennsylvania, and the other (H. horridum Cope), 

 being thus far known only from rocks of the same age in Brad- 

 ford county, Pennsylvania. One specimen of the typical species 

 is interesting in that it displays the entire ventral armor, and 

 from the relative proportion of its parts a theoretical associa- 

 tion with Mylostomids is considered justifiable, both for this 



* Williams, H. S., On the Fossil Faunas of the Upper Devonian. Bull. U. S. 

 Geol. Surv. no. 41, 1887, p. 43. 



