Freshwater Copepoda 95 
thirds the length of the furca, and the inner nearly as long as the furca. The 
receptaculum seminis is convex on its anterior margin. The spinous armature 
of the terminal segments of the exopodites of the swimming feet is represented 
by the formula 2,3, 3,3. Pl. III, fig. 12 shows a fourth foot. The first segment 
of the fifth foot is broad; the second segment is only slightly longer than broad, 
but varies somewhat in its relative dimensions; it is armed with a seta and a 
spine. This spine may be jointed, as in Pl. IV, fig. 1, or not separated from the 
segment, as in Pl. IV, fig. 5. 
Length of female, exclusive of furcal set, 1.85 mm. to 2.4 mm. 
This was found in collections from Collinson point, Alaska, made June 13, 
1914, Bernard harbour, June 18, 1915; Cape Bathurst, Northwest Territories, 
Canada, July 26, 1916; Chantry island, Dolphin and Union strait, Canada, 
June 17, 1917. 
Cyclops americanus Marsh. 
Cyclops americanus was found in only one collection, that made in a water- 
hole on the tundra at Cape Bathurst, July 26, 1916, one of the collections in 
which C. magnus was also found. 
DISCUSSION OF THE viridis GROUP. 
It is not intended to enter into a detailed discussion of viridis and the allied 
species, but a brief statement should be made to justify calling americanus a 
species rather than a variety, and to show the necessity of adding another to 
the long list of names which have been applied to these forms. Schmeil, 1892 
and 1898, brings all forms of this type under viridis. E. B. Forbes, 1897, in 
discussing American species, recognizes the species viridis, which includes 
C. ingens Herrick, with varieties brevispinosis Herrick, and insectus Forbes. 
Marsh, 1898, recognizes four varieties, ingens Herrick, brevispinosus Herrick, 
parcus Herrick, and americanus Marsh. Americanus Marsh is the insectus 
described by E. B. Forbes, and has the right of priority, as the original publica- 
tion of insectus was in such form that the species could not be recognized. 
Chambers, 1912, from a study of the chromosomes, concluded that C. viridis, 
C. americanus, C’. parcus, and C. brevispinosus should be considered as distinct. 
species. The recent work of the author has lead him to think that it would be 
much more convenient to consider them as species and avoid the use of a 
trinomial nomenclature, for these forms have characteristic habitats, and it is 
yet to be shown that they actually grade into each other. ; 
The differences between these species may be tabulated as follows:— 
Recep. sem. Furca. Sp. ter. seg. Sp. ter. seg. 
sw. ft. fifth ft. 
C. viridis....... Ant. margin concave. |Ciliated...... 2, 3, 3,3 {Not separated by joint. 
C. americanus. ../Ant. margin convex..|Not ciliated..| 3,4, 4,4 [Separated by joint. 
C. parcus....... Ant. margin convex. .|Not ciliated..| 2,3,3,3 [Separated by joint. 
C. brevispinosus..|Ant. margin convex. .|Not ciliated..| 3,4, 4,4 [Separated by joint. 
C. magnus...... Ant. margin convex. .|Ciliated...... 2,3, 3,3  |Either with or without joint 
It may be added that brevispinosus is a slender form, the spines of the swim- 
ming feet are unusually broad, the terminal segment of the endopodite of the 
fourth foot has a spine on its outer margin instead of the usual seta, and the 
outer terminal seta of the furca is reduced to a short broad spine. 
The original description by Herrick of ingens, in 1882, shows a figure of 
the receptaculum seminis with a concave anterior border. This, with very 
