1. The bucephalid cercaria from Crassostrea virginica (Gmelin), the commercial oyster of the Atlantic and Gulf coasts of North America, has been called Bucephalus haimeanus Lacaze-Duthiers and Bucephalopsis haimeana (Lacaze-Duthiers). Neither of these names is correct. Bucephalus haimeanus Lacaze-Duthiers, 1854, is a parasite of the European oyster, Ostrea edulis L. (and possibly of cockles, Cardium spp.) in oceanic or near-oceanic habitats, while the American species is a parasite of an estuarine oyster and is found almost entirely in the least saline parts of its host's range. The correct name of the American oyster bucephalid is Bucephalus cuculus McCrady, 1874.
2. The life cycles of all European marine bucephalids are still unknown. The life cycles described in the European literature are based entirely on morphological resemblances which are not close enough to be convincing, and none of them has been tested by experiment. Bucephalid cercariae do not show any of the features which are used to distinguish genera. So far as present evidence goes, Bucephalopsis haimeana is just as likely to develop into a Rhipidocotyle, a Bucephalus, or a Prosorhynchus as it is to develop into a member of the genus Bucephalopsis Nicoll, 1914, nec Diesing, 1855.
3. Bucephalopsis Diesing, 1855, is the name proposed by Diesing for a subgenus created especially for the cercaria Bucephalus haimeanus Lacaze-Duthiers, 1854. Nicoll had no right to use this name for the genus of gasterostomes which, as adults, have a muscular sucker at the anterior end and do not have accessory structures such as a hood or papillae. Up to the present, there is still no evidence that this genus has any connexion with Lacaze-Duthiers's cercaria, other than common membership in the family Bucephalidae. Therefore the new name Bucephaloides Hopkins has been proposed to replace the generic name Bucephalopsis Nicoll, 1914, nec Diesing, 1855, with Bucephaloides gracilescens (Rudolphi, 1819) as the type species. This genus does not include Bucephalus haimeanus Lacaze-Duthiers because the metacercaria and the adult form of that species remain unknown.
4. Tennent (1905, 1906, 1909) did not prove that there was any connexion between the oyster cercaria, Bucephalus cuculus McCrady, and the immature bucephalids in Menidia or the adult bucephalids in Strongylura. His drawings show that at least some of the bucephalids which Tennent studied had a hood and therefore belong in Rhipidocotyle. Re-examination of the bucephalids in Strongylura marina reveals that at least three species of bucephalids occur as adults in that host; these are described in this paper as Rhipidocotyle transversale Chandler, 1935, R. lintoni Hopkins, and Bucephaloides strongylurae Hopkins. Rhipidocotyle transversale and Bucephaloides strongylurae were also found in an immature (metacercaria) stage in Menidia, and were the only bucephalids found in Menidia during this study. The excretory systems of the three species in Strongylura all have features which exclude the possibility that they could develop from the oyster cercaria, Bucephalus cuculus.
5. Tennent's (1906) drawing (fig. 46) of an adult bucephalid from Strongylura marina, which Eckmann (1932) mentions as the best representation of the species characteristics of Bucephalopsis haimeana (Lacaze-Duthiers), was probably made from a specimen of Rhipidocotyle lintoni.
6. Tennent (1909) proved that eggs or larvae from an unknown adult bucephalid in Lepisosteus osseus could infect oysters, Crassostrea virginica, and develop into sporocysts which would grow in oysters for at least one month. This is the only experimental proof of a connexion between an adult bucephalid in a fish and a larval bucephalid in an oyster. Until the present, no bucephalid from a gar (Lepisosteus) had ever been described. In the present paper Rhipidocotyle lepis-ostei Hopkins is described from adults in Lepisosteus spatula, the alligator gar, in Louisiana. The metacercariae of Rhipidocotyle lepisostei are abundant in the fin rays of mullets, Mugil cephalus and M. curema. The excretory system of this species is not identical with that of Bucephalus cuculus, but is not so different as to exclude the possibility that the oyster cercaria might develop into R. lepisostei. If, in the future, R. lepisostei is proved to be the adult form of Bucephalus cuculus, the name of the species will become Rhipidocotyle cuculus (McCrady), and R. lepisostei will become a synonym.