ischiopterygium

(plural ischiopterygia))

pelvic fin (the paired fin which is located posterior, ventral or anterior to the pectoral fins (abdominal, thoracic or jugular in position). It functions to steer, brake and propel the fish and acts as a keel. In the pelvic fin ray count usually all the rays are counted except a small ray preceding the first ray and usually bound so closely to it so as to require dissection to be seen. In some fishes with reduced pelvics, the spine and the first ray may be bound together by a membrane and appear as one; both are counted, e.g. in Cottidae. Abbreviated as P2 or V)