Neotropical Monogenoidea. 29. Ancyrocephalinae (Dactylogyridae) of piranha and their relatives (Teleostei, Serrasalmidae) from Brazil: species of Amphithecium Boeger and Kritsky, 1988, Heterothecium gen. n. and Pithanothecium gen. n. Journal of the Helminthological Society of Washington, 64(1), January 1997: 25-54. 39 [Zoological Record Volume 133]