Abstract: Micro-structures of the valve, girdle and old valve jacket of Melosira arenaria Moore collected from the mudstone beds of Shanwang formation in Shanwang village, Linqu Xian, Shandong Province, PRC., have been observed under the optical microscope and SEM. The author found that the ornamentation of this diatom is quite similar on the girdie of different frustules, but rather variable on their valves. These variable structure can be classified into three types: 1. long striate type; 2. short striate type; 3, long wedge-shaped type. Furthermore, the fundamental structures of M. arenaria are very similar to that of M. teres, M. sol, and M. polaris. Thus we may consider that they are affinities in the phylogeny of algae.

Key words: Melosira arenaria, Old valve jacket, Girdle