What is "atrypa"

Atrypa \A*try"pa\, n. [NL., fr. Gr. 'a priv. + ? a hole.] (Paleon.) A extinct genus of Branchiopoda, very common in Silurian limestones.

Atrypa is a genus of brachiopod with shells round to short egg-shaped, covered with many fine radial ridges (or costae), that split further out and growthlines perpendicular to the costae and 2-3 times wider spaced. The pedunculate valve is a little convex, but tends to level out or even become slightly concave toward the anterior margin (that is: opposite hinge and pedicle). The brachial valve is highly convex. There is no interarea (that is a flat area bordering the hinge line approximately perpendicular with the rest of the valve) in either valve. Atrypa was a cosmopolitan and occurred from the late Lower Silurian ( Telychian) to the early Upper Devonian ( Frasnian).