308 
Birge—Notes on Clad'ocera. 
In all these cases, which include all the references I have been 
able to find, the cells are very shallow and their walls extreme¬ 
ly delicate, so that they are hard to see. They thus differ wide¬ 
ly in degree from the conspicuous cells of C. faviformis , which 
are the most noticeable facts in its structure. The walls of the 
cells stand at right angles to the surface of the shell, and ae 
this is nearly globular, the cells widen considerably toward 
the exterior. The cells are smallest in the anterior part of the 
valves just below the junction of the fornices,' and their walls 
are lowest at the same point. Here is the region where the 
antennas rub against the shell when reflexed. The largest cells 
are in the dorsal posterior region, where they may be 0.07 mm. in 
diameter and 0.05 mm. in depth. There is a row of smaller and 
somewhat irregular cells along the line of junction of the shell 
of head and body. 
The cells are often inhabited by a species of Vorticella, which 
when disturbed withdraws at once into the protection of the 
cavity. The animal is not as much overgrown with parasites 
as would be expected from the structure of the shell, Indeed,, 
very little was noticed except the Vorticella. Probably the fre¬ 
quent changes of the shell are the cause of this freedom from 
parasites. In examining the material collected by the dredge, 
far more cast shells of this species are found than of C. sphceri- 
cus, while the proportion of living animals is the other way. 
The post-abdomen and appendages show nothing peculiar. They 
closely resemble the corresponding parts of C. sphoericus. 
Chydorus faviformis lives in shallow water in lakes in north¬ 
ern Wisconsin. It was particularly abundant among a plentiful 
growth of Utricularia near the southern railroad trestle of the 
Milwaukee and St. Paul R. R., in the vicinity of Minocqua. 
Chydorus rugulosus, Forbes. 
Plate XIII, Fig. 6. 
A species of Chydorus apparently the same as that of Forbee 
was found at Washington Harbor, Isle Royale. It is probably 
also identical with Lilljeborg’s Chydorus gibbits (Sars, ’90, p. 50). 
