English: Protitanichthys sp. - fossil fish from the Devonian of Ohio, USA. (Dave Mielke collection; temporary public display, Ohio Geological Survey, Columbus, Ohio, USA)

The subcircular structure is an eye.

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From exhibit signage:

Arthrodires were a type of placoderm, early jawed fishes that became extinct before the end of the Devonian Period. The head and thoracic area of arthrodires and other placoderms were covered with a mineralized armor, while the rest of the body was naked or covered with scales. Some arthrodires grew quite large, like Dunkleosteus, which probably exceeded six meters in length and was the terror of Devonian seas.

Protitanichthys sp.

A nearly complete armored head shield of a large arthrodire placoderm fish. This individual Protitanichthys was probably about four feet in length. (quite rare)

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Classification: Animalia, Chordata, Vertebrata, Placodermi, Arthrodira, Coccosteidae

Stratigraphy: Silica Formation (also known as the Silica Shale), Givetian Stage, upper Middle Devonian

Locality: quarry northwest of the town of Paulding, northern Paulding County, northwestern Ohio, USA (41° 10' 52.55" North latitude, 84° 37' 19.32" West longitude)

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See info. at: en.wikipedia.org/wiki/Protitanichthys and en.wikipedia.org/wiki/Arthrodira and

en.wikipedia.org/wiki/Placodermi