The generic term Diphyllobothrium was coined by Cobbold (1858) for five tapeworms taken from a porpoise, Delphinus phocaena, in Scottish waters, and was eventually adopted by Liihe (1910) in place of his previously (1899) suggested generic term Dibothriocephalus for a group of forms centering around the Linnaean species latus. It is a cumbersome group of about 70 species-many of them of dubious validity-and comprises forms from toothed whales, seals, sea-lions, carnivorous land mammals and fish-eating birds. Several species have been recorded from humans and one even from a snake. It has always been an unsatisfactory genus to define and analyze, and particularly difficult to evaluate have been the forms from seals and sea-lions that have been recorded by numerous writers. An examination by the writers of a range of such material from seals in Canadian Atlantic waters and from sea-lions in Alaskan waters uncovered the following types. 1. Short muscular forms with the scolex short, thick, heart-shaped in marginal outline and up to 500 / wide, triangular or oval in surficial outline (bothrial view) and up to 300 p wide. Bothria deep, gaping anteriorly, narrower posteriorly, so as to appear keyhole-like, and extending the full length of the scolex, with margins prominent. No glandular tissue seen within the scolex parenchyma. Neck short, hidden usually by the overhang of the scolex. Body short, readily fragmenting and consequently difficult to estimate in total length; up to 6.5 mm wide and 1.25 mm thick. Segments extremely short and craspedote. Strobilar margins markedly serrated. Cirro-vaginal pore of the relaxed segment at the junction of the first and second thirds of the ventral surface, in the contracted segment being nearer to the anterior border and hidden usually by the overhang of the preceding segment. Parenchymal muscle zone well developed, appearing in a cross section as a conspicuous, deeply staining, continuous band, as thick dorso-ventrally as the enclosed medulla. Two sets of genitalia per mature segment. Testes large, reaching from floor to ceiling of medulla, appearing in a cross section of the segment as a single layer of 15-20 follicles between each margin and the nearest uterus, and as a double or triple layer of 10-12 follicles between the two uteri. Each cirrus pouch one-third as long as the medulla is thick, with a dorsal, external seminal vesicle, half its length. Yolk glands forming an almost uninterrupted band in the cortex, thinning, or very occasionally lacking, opposite the distended uteri. Each uterus with 3-4 loops on each side, closely appressed and parallel, the terminal loop distended and level with the cirrus pouch. Intra-uterine eggs averaging 50 by 40 p, with rounded ends. Adults in seals (Phoca groenlandica, Erignathus barbatus) in Labrador waters. This type recalled the descriptions of Diplogonoporus fasciatus (Krabbe, 1865). 2. Short muscular forms with the scolex also short and thick, heart-shaped or broadly oval or rectangular and 400-1000 I wide in marginal outline, triangular or lanceolate and 800--1400 ~ wide in surficial outline (bothrial view). Bothria deep, slit-like or keyhole-like, gaping anteriorly, narrowing posteriorly. No glandu-