Abstract
SEM observations are reported on a naviculoid raphe-system intricate with several accessory components: shark fin-like vanes here termed dorsicentral and subcaudal fins, dorsal ridges of keel, a longitudinal groove. between the ridges and helictosiphons at poles in addition to the ordinary raphe on highly vaulted valves. Four new species are described under Dichotropiscus, a new genus, characterized by sharing this system. They are D. fimbriatus, D. bicostatus, D. brevilanceolatus and D. staurophorus. Morphological details and functional accounts are given in relation to the fins.
