Freshwater Copepoda 73 
is shown in Pl. III, fig. 5. Schmeil, 1893, has shown that there may be much 
variation in the length of this appendage. 
The male fifth foot is shown in Pl. III, fig. 4. The second basal segment of 
the right foot has the small lateral seta near the distal end of the segment; from 
about midway of the inner border projects a hyaline lamella, which extends 
over the central third of the segment. The first segment of the right exopodite 
is sharply produced at its outer distal angle. The second segment is not quite 
twice as long as its greatest width. The lateral spine is straight, acute, nearly 
as long as the segment, and placed just beyond the middle. The terminal hook 
is sickle-shaped and slender. Some authors say that there is a projection or 
spine on the dorsal surface of the second segment of the exopodite; this is shown 
by Koélbel, but not by Sars, 1903. Schmeil, 1893, says that it is variable, being 
sometimes present and sometimes absent. The second basal segment of the left 
fifth foot of the male has the small lateral hair well towards the end of the segment. 
At about the middle of the inner border is a small hyaline lamella, and from the 
inner distal angle extends distally a cuticular, tooth-like projection. The 
terminal segment of the left foot has a digitiform process and a curved spine. 
The endopodites are indistinctly two-segmented. Kélbel and Sars, 1903, say 
that the endopodites are one-segmented, but DeGuerne and Richard, 1889, in 
their figure, make them indistinctly two-segmented. DeGuerne and Richard 
state that the left endopodite of the male fifth foot is not separated by a joint, 
and the figure of Sars, 1903, shows the same condition. Kélbel, however, figures 
a joint, as does Sars, 1898. 
It is in the structure of the second basal segments of the male fifth feet 
that we find the characteristics which are most diagnostic of this species of 
Diaptomus. They were not figured fully by Kélbel, but they appear in the later 
authors with a rather surprising lack of variability. This lack of variability is 
the more surprising because of the marked resemblance in the general structure 
of the male fifth feet in D. bacillifer Kélbel, D. lattceps Sars, D. salinus Daday, 
D. acutilobatus Sars, D. wierzejskit Richard, D.hircus Brady, and D. similis 
Baird, as defined by Richard. 
D. bacillifer has been found in Scotland, Norway, many places in the Alps, 
Asia Minor, Syria, the Caucasus, India, Central Asia, Siberia, and in islands 
north of Siberia. It is a stenothermal cold-water form, and is found in the far 
north in bodies of water near the sea level, and farther south in lakes in the 
higher mountains. 
In the collections of the Canadian Arctic Expedition it was found only in 
the gathering. made on October 6, 1915, from a pond one foot deep a hundred 
feet above sea level on a ridge at Bernard harbour. Some Diaptom collected 
on St. Paul island, Alaska, by Professor Parker, were sent to the author some 
time ago, and proved to be of this species. Apparently then, it encircles the 
world in the general neighbourhood of the Arctic circle, and probably will be 
found in many of the bodies of water in northern Canada. It seems strange 
that it has not appeared in the collections which have been made in Iceland and 
Greenland. 
D. bacillifer has the distinction of having a wider distribution than any other 
species of the genus. 
Diaptomus arcticus, n. sp. 
Plate III, figs. 6-9 and 13. 
This large and conspicuous species is closely related to D. shoshone, so closely 
in fact, that it can only be distinguished from that species by details of the 
structure of the abdomen and fifth feet. 
The first abdominal segment of the female is dilated laterally and in front 
and bears on either side a blunt spine. This is shown from the side in PI. III, 
fig.6. The antenne equal in length the cephalothorax. In the fifth foot, 
