Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
prōsūmĭa, ae, f., a small kind of vessel for reconnoitring, a spy-boat, Caecil. ap. Non. 536, 11 sq. (Com. Rel. pp. 29 and 45 Rib.); cf.: prosumia genus navigii speculatorium parvum, Paul. ex Fest. p. 226 Müll.