Aulacoseira herzogii

Frustules are cylindrical and form short colonies, usually of two frustules. Valves are 3-7 µm in diameter and have a mantle height between 7-14 µm. The ratio of mantle height to valve diameter is either equal to, or greater than, 1. The mantle is straight and the valve face is either flat or slightly radially undulate with a concave center. Small areolae are present on the valve face. The straight rows of mantle areolae are often not resolvable in LM and number 30-32 per 10 µm. These small areolae, if visible in the light microscope, appear circular, although they are irregularly shaped under SEM and number 27-37 per 10 µm along the pervalvar row. Two diametrically opposed grooves, containing no areolae, extend the entire length of the valve. Two large, pointed separation spines diametrically opposed on the valve face extend in the plane of the mantle onto grooves of the neighboring frustules. The planes of the large spines and the grooves are at right angles to each other. Smaller, broadly pointed spines are present at the end of each costa. No valves with linking spines have been observed. The ringleiste is relatively shallow. According to Hickel & H. Håkansson (1991) the number of spines may be 2-4, but specimens that we observed from the U.S. had 2 spines.