_Cirrus sac usually reaching to centre of ventral sucker, but not beyond. Cirrus long, usually without spines, coiled when retracted. Seminal vesicle tubular, twisted. On the head a ventral uniting ridge between the angles of the collar. Dorsal circlet of spines, single or double, not interrupted unless the collar itself is dorsally divided. Genera_: Echinostoma, etc. Sub-family. *Himasthlinæ*, Odhner, 1910. Cirrus sac reaching far beyond ventral sucker. Cirrus armed with strong rose-thorn-shaped hooks. Vesicula seminalis tubular not coiled. Cervical collar not continued across ventral aspect. Spines on collar in one row. Body armed with fine needle-shaped spines. Family. *Schistosomidæ*, Looss, 1899. Sexes separate. Genital pore behind the ventral sucker. Ventral sucker elevated above the surface. Pharynx absent. Gut forks reunite to form a single stem. In ♂ four or more testicular follicles. In ♀ a single ovary, just in front of the union of the gut forks. Vitellaria on either side of the united gut stem. THE TREMATODES OBSERVED IN MAN. Family. *Paramphistomidæ*, Stiles and Goldberger, emend. 1910. Sub-family. *Cladorchiinæ*, Fisch., 1901. Genus. *Watsonius*, Stiles and Goldberger, 1910. _Cladorchinæ_.--Body pyriform. Ventral pouch absent. Acetabulum ventral or (?) ventro-subterminal, very large, margins projecting, aperture small. Genital pore in front of bifurcation of gut, not surrounded by a sucker; ductus hermaphroditicus apparently absent. Excretory pore at posterior end of excretory vesicle, behind Laurer’s canal. Oral sucker with a pair of irregularly globular suctorial pouches; œsophagus thickened distally; cæca long, not wavy; end in acetabular region. _Male Organs_.--Testes two lobed, smaller than acetabulum; longitudinally, nearly or quite coinciding; transversely they abut or slightly overlap; preovarial in equatorial and caudal thirds. Pars musculosa not largely developed; cirrus pouch absent. _Female Organs_.--Ovary and shell gland post-testicular. Vitellaria extend from gut fork to slightly beyond gut ending; uterus intercæcal, partly post-testicular. Laurer’s canal in front of excretory vesicle. _Type Species_.--_Watsonius watsoni_, Conyngham, 1904. *Watsonius watsoni*, Stiles and Goldberger, 1910. Syn.: _Amphistomum watsoni_, Conyngham, 1904; _Cladorchis watsoni_, Shipley, 1905.