The Hiriulinca of Illinois. 495 



complete somites a 2 is somewhat wider than its fellows, 

 dorsally at least, and is rendered further conspicuous owing 

 to the papilla which it alone hears, its slightly protuherant 

 margins, and the white spots which characterize it. 



The arrangement of the cutaneous papillae, though variable 

 in detail, is very characteristic of the species among local 

 forms. But three constant and conspicuous series exist 

 (PI. XLIL, Fig. 6) ; a median {mp) and a pair of dorso- 

 laterals {(Up) situated nearly half-way to the margins. The 

 median series begins sometimes as far forward as somite 

 YI or YII, but seldom becomes prominent before IX or X, 

 from which it continues to XXVI. This is really not a strictly 

 median series, as the papillae comparatively seldom lie exactly 

 in the median line but usually a little to the right or the left. 

 Very frequently the single conical papilla is replaced by two 

 which are placed more or less close together, in contact or even 

 united into a bilobed or a transversely elongated one. The 

 whole appearance of the series must lead one to recognize its 

 dual character. All of these papillae are prominent, sharp- 

 pointed, simple cones, wdiose black color amid white or pale 

 surroundings renders them doubly conspicuous. They are 

 largest in the middle of the series and diminish toward the 

 ends, that on XXVI being very small and some of the 

 anterior ones minute. 



In every important feature the dorso-lateral series is 

 similar to the median. The papilhi^ begin further back and 

 are seldom conspicuous anterior to XIII, from which point 

 they continue to XXVI or XXVII, the last two or three being 

 small. They are somewhat smaller than the median ones 

 and are more likely to be absent from a somite. Though 

 forming a nearly perfect series anteriorly, they present an 

 even greater tendency toward irregularity and doubling 

 posteriorly. 



Occasionally one or two isolated papillae are found between 

 the median and dorso-lateral series, and remnants of a very 

 imperfect supra-marginal series occur more frequently on 

 several of the posterior somites. Very few and faint papillae 

 occur on the sucker. The dorsal surface of the body 



