Freshwater Copepoda 



9J 



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 setae, 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 VlTldlS 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 Herriok, 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: 



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 



