Δελφῖνα νήχεσθαι διδάσκεις: ἐπὶ τῶν ἐν ἐκείνοις τινὰ παιδοτριβούντων, ἐν οἷς ἤσκηται → Teaching dolphins to swim: is applied to those who are teaching something among people who are already well versed in it
sēmĭ-circŭlus: a, um, adj.,
I semicircular: ager, Col. 5, 2, 8.—Hence, subst.: sēmĭ-circŭlus, i, m., a half-circle, semicircle (syn. semiorbis): uncus in semicirculi speciem, Cels. 7, 26, 2: ager, i. e. semicircular, Col. 5, 2, 8.