The state of knowledge of Australian fish parasitic ascardioids is briefly reviewed. The following species are described in full: Hysterothylacium cornutum, H. pelagicum, H. scomberoidei n. sp. Additional data are provided for Iheringascaris inquies. Maricostula n. gen. is established for those species that have lips with lateral flanges, deep postlabial grooves, interlabia, and elongate intestinal caecum, pyriform ventriculus, long sac-like ventricular appendix, an expanded filamentar excretory system and excretory pore near the nerve ring. The males of all the species have precloacal crests. Species included are: M. caballeroi (Deloya), M. cenatica n. sp., M. histiophori (Yamaguti), M. incurva (Rudolphi), M. makairi n. sp. and M. tetrapteri n. sp. All are recorded from billfishes, and all but M. caballeroi are recorded from Australian waters. The systematic position of the genus is discussed. New characters of taxonomic utility used in the discrimination of fish parasitic ascaridoids are discussed. These include fine details of alae, male caudal crests and papillae. We demonstrate that these characters can be used to separate species of Maricostula. Remarks are also given on nomenclatural problems within the Indo-Pacific fish parasitic Ascardioidea.
