DEVONIAN FISHES OF IOWA 85 



consisting of fine rounded tubercles fused into nodose, vermicu- 

 lating ridges; those near the edges of the dorsal plates often 

 displaying concentric arrangement. ("Woodward.) 



The following technical account of the structure of this ex- 

 tremely important species has been compiled chiefly from the 

 recent papers of Traquair and Patten, cited in the above refer- 

 ences to the literature. 



The headshield occupies nearly one-third of the entire length 

 of the armoring, and shows, on the upper surface, an orbital 

 opening which is smaller and further back than in Asterolepis. 

 The median occipital plate (text-fig. 12 m. occ.) has its lateral 

 margin more perpendicular to the posterior one than in the 

 last-named genus ; its anterior aspect shows not merely a shallow 

 re-entering angle for the postmedian plate, but a deep semi- 

 elliptical notch or excavation, on each side of which it takes part 

 in the formation of the posterior boundary of the orbit. Con- 

 sequently the postmedian plate is small, entirely received in the 

 aforesaid notch of the median occipital, and thus excluded from 

 joining the laterals as in Pterichthys and Asterolepis. The lat- 

 eral occipitals (I. occ.) and the angular (ag) do not call for any 

 special comment, but the laterals (I) are much broader than 

 in Asterolepis, while the extralaterals (e. I.) are very small, nar- 

 row, and pointed in front. 



The orbital opening (o) is, as already mentioned, small com- 

 pared with that of Asterolepis, and moreover, its anterior mar- 

 gin shows scarcely any re-entering flexure. Its right and left 

 portions are almost completely filled by a system of sclerotic 

 plates, the two inner ones being very considerably larger than 

 . the two outer; and the middle portion is covered by two loosely 

 attached plates, namely, the median or pineal (m), and a very 

 narrow T-shaped plate (called by Patten the "ethmoid") close 

 in front of it. From the center of this narrow plate, as shown 

 by Whiteaves and confirmed by Patten, a small linear process 

 with expanded lower extremity passes down perpendicularly into 

 the interior of the head, almost reaching a thin transverse shelf 

 of bone which projects downwards from the under side of the 

 premedian (p. m.). The space thus partitioned off beneath the 

 premedian is interpreted by Patten as an "olfactory chamber", 



