308 Birge — Notes on Cladocera. 



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 as 

 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 sphwri- 

 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 E. R., in the vicinity of Minocqua. 



Chydorus rugulosus, Forbes. 



. Plate XIII, Fig. 6. 



A species of Chydorus apparently the same as that of Forbes 

 was found at Washington Harbor, Isle Royale. It is probably 

 also identical with Lilljeborg's Chydorus gibbus (Sars, '90, p. 50). 



