1. The author of some medical works, of which nothing but a few fragments remain, who quotes Archigenes (ap. Aet. 2.2. 12, p. 255), and is himself quoted by Rufus Ephesius (ap. Ang. Mai, Classic. Auctor. e Vatic. Codic. Edit. vol. iv. p. 11), and who must, therefore, have lived about the end of the first century after Christ. He is one of the earliest writers who is known to have mentioned the glandular or true plague, though this disease was, till quite lately, supposed to have been unknown till a much later period (see M. Littré, loco cit.). He is several times quoted by Aetius (1.3. 121, 2.2. 2, 3, 5, 6, 7, 8, 10, 12, 13, 18, 20, 21, 24, pp. 139, 243, 244, 246, 247, 248, 254, 255, 257, 258, 260), and Paulus Aegineta (7.3, 21, 22, pp. 614, 692, 693). The name frequently occurs in Galen, but it is probable that in every passage the philosopher is referred to and not the physician. If (as seems upon the whole not unlikely) this Poseidonius is the pupil of Zopyrus at Alexandria, who is mentioned by Apollonius Citiensis as his fellow-pupil (ap. Dietz, Schol. in Hippocr. et Gal. vol. i. p. 2), there is a chronological difficulty which the writer is not at present able to explain.