Prosternum in front of coxae concave or biconcave, or flat to moderately convex. Erythrocytes are in the shape of a biconcave disc. Ribs are falcoid or falcate and thus biconcave, strong and projected. A lens with two concave surfaces is "biconcave" (or just "concave"). Parasitized red blood cells often lose their biconcave shape. The red blood cell is highly flexible and biconcave in shape. It has two biconvex lens on the outer and a biconcave lens in the middle. The abnormal erythrocytes are sphere-shaped (spherocytosis) rather than the normal biconcave disk shaped. In other trematopids, the margin is biconcave. In humans, mature red blood cells are flexible and oval biconcave disks. The platelets will transform from smooth, biconcave discs to fully spread cells. The middle contains a biconcave core which appears to be characteristic to many poxviruses. The resulting two biconcave elements are closer together than in the Dialyte/Celor design. It is biconcave in structure and attaches to the condyle medially and laterally.