Neithea irregularis

These species have a shell that is usually about 17mm x 14mm x 5mm.  The shells range from small to medium; they are subtrigonal, more or less elongate, almost equilateral with a polygonal ventral margin, and the ears are large and hinge line straight.  Both valves are decorated with 16-17 radiating ribs.   These species can be found in the Glen Rose Formation, Mt. Bonnell; and Walnut Clay, Mt. Barker, Austin, Texas.