48 Illinois State Laboratory of Natural History. 



SYNONYMY. 



Since the description of C. gyrinus by Dr. Forbes in 1890, 

 more careful descriptions of C. albidus in Europe and fur- 

 ther studies in America have established the identity of the 

 two forms. The original description of C. gyrinus was in- 

 complete in that the presence of the sense-club and hyaline 

 plates was not observed. 



SPECIFIC DESCRIPTION. 



This is a stout, heavy species, with a strongly arched 

 cephalothorax, which, as a whole, is usually quite elliptical, 

 with the lateral angles of the segments almost invisible ; 

 but in specimens from Manitoba I find the cephalothorax 

 shaped much as in C. viridis. The first segment in these is 

 subspherical, narrowing posteriorly, and the posterior end 

 of the second segment is much broader than the anterior 

 end. All of the lateral angles are prominent. The breadth 

 of the first segment compared with its length is as ten or 

 eleven to twelve. 



The dorsum of the fifth segment in this species is or- 

 namented by three or four transverse rows of spinules, the 

 posterior one of which borders the segment. 



The abdomen is thick and heavy. The first segment in the 

 female is but slightly enlarged and tapers very little. The re- 

 maining segments are cylindrical. The stylets are short and 

 slightly divergent. The proportion of the length to the 

 breadth of the stylet varies but slightly. The usual ratio is 

 two and one half to one. The apical bristles are all well 

 developed, the third from without being the longest, and the 

 innermost three times as long as the outermost. 



The first antenna? of the female are seventeen-segmented, 

 and reach to the first abdominal segment. The distal three 

 segments bear hyaline plates, entire, except for minute ser- 

 rations on the distal half of the plate of the seventeenth seg- 

 ment. The twelfth segment bears an unusually large sense- 

 club. 



The swimming feet are armed as follows : — First pair : 

 outer ramus, four spines, four setae; inner ramus, one seta, 



