1) Animal order 2) Suborder heterostraci 
(from the article `fish`) Heterostracan remains are next found in what appear to be delta deposits in two North American localities of Silurian age. By the close of the ... ...cm (roughly 6 to 40 in.).†Subclass DiplorhinaWith 2 nostrils.†Order HeterostraciOrdovician to Upper Devonian. Usually heavily armoured, with a ... .....
[n] - extinct group of armored jawless fishes or fishlike vertebrate
suborder Heterostraci noun extinct group of armored jawless fishes or fishlike vertebrate; taxonomy is not clear