Muelleria gibbula

Valves are elliptic-lanceolate to linear in the largest specimens. The central margin is typically gibbous although some specimens lack a central inflation. Apices are broadly rounded. The raphe is straight and situated between two longitudinal canals. The axial area is narrow and linear, widening slightly near the small, elliptic central area. Proximal raphe ends are unilaterally deflected. Internally, a rectelevatum is present between the two raphe branches, typical for the genus Muelleria. Striae are radiate throughout, the central striae more widely spaced than in the rest of the valve. Areolae in the central striae appear larger and more distinct than areolae in more distal striae. Copulae are evident in girdle view in some specimens. External SEM images show simple round areolae openings, unilaterally bent proximal raphe ends that do not enter the first row of areolae, and bifurcating distal raphe fissures that separate at a wide angle due to the position of a large siliceous flap.