Cosmarium ovinum nom. et stat. nov., een nieuwe naam voor een kleine op C. decedens lijkende vorm

Cosmarium ovinum nom. et stat. nov., a new name for a small form resembling C. decedens. In a sample of a drying rain puddle in a sand drift in nature reserve “Schaopedobbe” (province of Friesland, the Netherlands) a small form resembling C. decedens was found. The form was initially identified as C. decedens var. apertum and also compared with var. sinuosum. Detailed taxonomic research, however, indicated that these varieties are highly problematic and that the present material more closely resembles the poorly known C. decedens var. parallelum. The cells showed great variability with simple, subquadrate semicells to semicells with more prominent, extended angles resembling very small forms of C. decedens. However, dimensions were always smaller than known for last mentioned species and in my opinion it cannot be considered a mere “Kümmerform”. These characters induced me to describe this variety here as a separate species Cosmarium ovinum as a replacement name for Cosmarium decedens var. parallelum E.A. Messikommer 1945, Mitteilungen der Naturforschenden Gesellschaft in Bern, p. 101, Pl. 2: 42. In addition, an emended description is given: cells in outline rectangular, about twice as long as wide. Sinus very shallow and more or less wide open with a narrowly rounded apex. Basal angles of semicells with variable shape, from broadly rounded to narrowly ronded and slightly drawn out, sides rather straight and more or less parallel, apical angles broadly rounded and not or only slightly drawn out. Apex generally less wide than the basis and markedly retuse. Lateral view of semicells broadly elliptic with a straight basis, apical view more or less lemon shaped depending on the development of the basal angles. Cell wall ornamented with rather coarse pores. Chloroplast axile with a single pyrenoid. Dimensions: length 27.5-33.2 µm, width 13-16 µm, thickness circa 12 µm, isthmus 10.5-13.5 µm, length/width 1.9-2.3.