380 Illinois State Laboratory of Natural History. 



Phelps Lake. It also appeared in the river at E in August 

 and September, 1894, becoming very abundant in August. 



This is a very difficult species to work with, since it varies 

 greatly. Specimens were found ranging from .17 to .25 

 mm. in length. The pectoral edge is slightly more notched 

 in our specimens than in the typical form. 



91. B. variabilis Hempel ('96). 



This is a very restless active species, most common from 

 April to July, but present also in most of the other months 

 of the year. It was found only in the open water, but at 

 nearly all of the substations, and was very abundant in 

 Phelps Lake at the time when the Station work first began. 



92. B. bakeri Ehrbg. 



This is preeminently a summer species, making its first 

 appearance in our collections in May, becoming common 

 from June to August, and disappearing again in October. It 

 occurred in all the waters studied except Flag Lake, and was 

 found among vegetation as well as in the open water. 



This is also a variable species. 



93. B. bakeri brevispinus Ehrbg. 



Infrequent. Found in towings from the Illinois Eiver 

 during April, 1898. 



94. B. angularis Gosse. 



This species was common and often abundant in summer. 

 It occurred from April to September, with occasional appear- 

 ances in October, December, Fel)ruary, and March, its time 

 of greatest abundance being in July and August. It was 

 found in all of the waters of the Station except Flag Lake. 



Comparatively few specimens were found that had the 

 characteristic bluntly angled lorica. The majority of them 

 were more or less smooth; and in many instances there were 

 minute dots or tessellations on the surface of the lorica. In 

 connection with this species was found the following variety 

 with two posterior spines. 



