
890 THE AMERICAN NATURALIST. [VoL. XXXVI. 
Subfamily II. Diplostominz Brds. 
Genera: Diplostomum, Polycotyle. 
Subfamily III. Hemistominz Brds. 
Genus: Hemistomum. 
Subfamily IV. Holostominz Brds. 
Genus: Holostomum. 
Family V. Gasterostomide Brn. 
Genus: Gasterostomum. 
Family VI. Didymozoénide Mont. 
Genera: Didymozoón, Nematobothrium. 
Family VII. Monostomidze Mont. 
Subfamily I. Microscaphidiinz Lss. 
Genera: Microscaphidium, Deuterobaris. 
Subfamily II. Pronocephalinz Lss. 
Genera: Pronocephalus, Pleurogonius, Glyphicephalus, Adenogaster, Crico- 
cephalus, Pyelosomum. 
Related genus: Charaxicephalus. 
Subfamily III. Haplorchidinz Lss. 
Genera: Haplorchis, Galactosomum. 
Additional genera: Opisthotrema, Cycloccelum, Notocotylus, Ogmogaster, 
Stictodora, Mesometra, Monostomum. 
ORDER. — TREMATODA RUD. 
Small parasitic flatworms, with unsegmented, flattened or cylindrical, 
unciliated bodies, with usually anterior mouth opening and bifurcate intes- 
tine, and without anal opening, which attach themselves to their hosts by 
means of suckers, or hooks, or both 
KEY TO THE SUBORDERS. 
A,. Usually ectoparasitic trematodes living upon the external surface or 
the gills, or in the mouth or cloaca of aquatic animals (except genus 
Polystoma), to which they attach themselves by means of suckers, 
or hooks, or both ; suckers, when present, are usually near either 
one or both ends of the body ; when at the anterior end, in most 
cases, a single pair is present; when at the posterior end, in most 
cases, one or more pairs are present, or, in their place, a sucking 
disk . . Heterocotylea Mont. 
A, Endoparasitic PRSE viti ‘wes heic to their hosts either 
by means of one or more median (unpaired) suckers or a large 
ventral sucking disk; hooks never present. 

