24 BRISTOL. [V^OL. XV. 



less and less marked. At the time of greatest distinctness 

 one may see that the sensillae are rather more strongly marked 

 on the terminal somites, especially those at the hind end. It 

 was here that I was able to find a starting point for determin- 

 ing the external metamerization. The 97th ring (PI. VI, Fig. 3) 

 was strongly marked with sensillae, and between the 96th and 

 97th rings were found the pores of the 17th and last pair of 

 nephridia. These two conditions gave me a starting point 

 from which I could fix with certainty the limits of the somites 

 towards the anterior until the reduced somites of the eye region 

 were reached. The nephridial pores were used as the limiting 

 marks of the somite forward to the first pair of pores which lie 

 between the i6th and 17th rings. From this point forward 

 the sensillae aided somewhat, but the final results were based 

 on the distribution of the nerves. 



A careful reexamination of my material now showed that, 

 with one exception, I had collected or examined but one spe- 

 cies of Nephelis. The exception was found among the leeches 

 collected by Forbes in the Yellowstone region, and while the 

 differences are such that I feel warranted in suggesting that 

 proper study may show them to belong to another species, I 

 could not, from the specimens at hand, determine this point. 



The common species of Nephelis found east of the Rocky 

 Mountains is the one that I have used in my investigations. 

 The names adopted by Verrill (3) must, as he prophesies, be 

 abandoned, and the name Nephelis lateralis be retained for this 

 species so widely distributed over the United States. 



Description. 



The size of the sexually mature adult varies from 4 cm. to 

 10 cm. at rest. Anterior to the sexual openings the body tapers 

 gradually to the mouth ; posterior to them the body continues 

 about the same size until a little in front of the anus, where 

 it narrows to the sucker. The transection of the body is 

 lenticular, though in the pre-clitellar region it approximates 

 a circle. The body flattens in swimming as it does in Macro- 

 bdella and Hirudo. 



