6 30 Illinois State Laboratory of Natural History. 



metameric sensillse in surface views the metamerism is less 

 readily determined. Comparison with //. marmoratis hrings 

 to light some interesting points of distinction between the 

 two species. No differences of any consequence are notice- 

 able in the first five somites. 



In VI, a 3 is relatively much longer, and occasionally a 

 faint subdivision appears on its dorsal surface. 



VII presents a similar condition in a 1, while a 3 is repre- 

 sented by the fully developed secondary annuli b5 and b6, 

 which are completely separated both dorsally and ventrally, 

 making this somite quadriannulate, with the formula 

 al+a 2+£5+£ 6, or [b l+£2)+a*2+£5+£6. 



VIII is quinqueannulate, owing to the complete separa- 

 tion of b 1 and b 2. This species presents, therefore, one 

 more complete somite — having the formula b l-\-b 2+#2-r 

 b 5-\-b 6 — than does H. marmoratis. 



In most of the complete somites, usually from about X to 

 XXI, a characteristic relative size of the component annuli 

 is maintained. .42 is always the shortest, b\ and £2 are 

 equal and slightly longer, and b 5 and b 6 are equal and still 

 longer. The faint depressed line which is mentioned above 

 as crossing most of the annuli is rarely discernible on a 2, 

 while on the secondary annuli it is usually quite evident. 



In XXIV, which is the last complete somite, the relative 

 size of the annuli anterior and posterior to the neural annulus 

 (a2) is reversed, bl and b 2 being larger than b5 and b6. 



XXV is triannulate, a 3 being distinctly smaller than a 1 

 or a 2, especially on the ventral side, where it becomes some- 

 what approximated to a 2 and the dividing furrow less deep. 



XXVI is biannulate, the first annulus being dorsally as long 

 as the second at the margins, or even longer, but relatively 

 smaller mesially. By displacing the sucker the second ring is 

 seen to include on each side a remnant of a very narrow 

 posterior ring, which in one specimen is well developed both 

 dorsally and ventrally. As the supposed sensilla? are found 

 on the anterior part of the second ring this is regarded as 

 representing a 2 and ad. 



XXVII is biannulate and includes the anus. 



