Guide to Auricula

Auricula includes a small, but uncertain, number of epipelic (living on sediment) taxa of marine habitats.

Paddock and Sims (1980) studied A. amphitritis and emended the genus description. They considered that, at most, only 12 previously described species possessed characters in common with the generitype and, therefore, are valid Auricula taxa. Since then, two of these taxa have been transferred to the genus Protokeelia by Round and Basson (1995) and two new species have been described by Osada (1997) and Zheng et al. (2022). 

Frustules are solitary. They are broadly oblong or nearly square with rounded angles, constricted in the wide girdle region, and dorsiventral in transapical section. Valves are crescent-shaped or bilobate, with a strongly eccentric raphe system. Valves are symmetric only to a plane through the central nodule.  The dorsal valve margin is convex and the ventral margin is concave or straight, with bluntly rounded apices. The eccentric raphe is on a raised keel at the junction of the valve face and mantle. The valve mantle is well developed only along dorsal margin of valve. The valve face is flat or slightly undulate. The canal raphe system is biarcuate, fibulate, and extends the entire length of the dorsal margin. Individual fibulae are short and rib-like. There is a prominent sternum on each side of the raphe from which virgae arise directly opposite one another that project above the valve face and mantle both internally and externally. External proximal raphe fissures are simple, straight, and interrupted at mid-valve by the central nodule. Distal raphe ends lack terminal fissures, but may extend well onto ventral valve margin, so that they closely approach one another. Biseriate striae are present between adjacent virgae with a thin strip of silica separating two rows of small, round poroid areolae that appear to be hymenate. Virgae may frequently bifurcate.

The girdle is wider on the convex dorsal side of the frustule than on the concave ventral side and this is also true for the individual girdle bands. The bands follow the curvature of the valve margin and are numerous, open, and strap-like. The region of overlap is restricted to the ventral side of the frustule beneath the valve poles. The advalvar margin of each band possesses a single, transverse row of small, round poroid areolae whereas the abvalvar margin has pervalvarly oriented biseriate striae where a thin strip of silica separates the two rows of poroid areolae. Osada (1997) provides a more detailed description of the girdle bands of Auricula densestriata.

Living cells possess a single, large, plate-like, lobed plastid that lies along the ventral side of the frustule girdle. A single, central, elongate pyrenoid is present in the plastid.

Auricula is superficially most similar to the genus Thalassiophysa. It can easily be distinguished from Thalassiophysa by its possession of prominent transapical virgae between the striae which are lacking in the latter, and the absence of a spur-like extension of the central nodule termed a mucro in Thalassiophysa. 

Some species of Auricula could conceivably be confused with those of Rhopalodia (Epithemia)  In the former genus the pars interior of the valvocopula underlaps only the valve margin whereas in the latter the advalvar margin of the valvocopula is extended as tongue-like structures that overlap the internal virgae of the valve (Paddock and Sims 1980). Furthermore, unlike Auricula, the areolae of Rhopalodia are occluded by volae and the canal of its more complex raphe system communicates with the cell interior via rounded portulae which lie between the strongly developed transapical costae (i.e. fibulae). 

Auricula is also similar to Entomoneis but can be distinguished by the strongly eccentric raphe of the former and the often laterally compressed frustule of the latter.