XXVI.-SYNOPSIS OF THE NORTH AMERICAN FRESH-WATER 



i.EECHES. 



By a. E. Vekrill. 



The leeches are rehited to the fisheries in three wi\ys. Some of the 

 large blood-sucking species, like Maorobdella decora and the species of 

 Hlrudo, attack many fishes directly, even when of considerable size, 

 and destroy them very qnickly by sucking their blood; and the species 

 of Icthyohdella and Cijstobranchus are trne parasites of fishes, and often, 

 when numerous, do them much injury. Other kinds, like the various 

 species of Clepsine, N'ephelis, Aulastoinum, &c., destroy all sorts of small 

 mollusks and worms, which otherwise might become the food of fishes. 

 But, on the other haud, certain kinds of leeches are fed upon, to some 

 extent, by the lake white-fish and probably by other fishes. 



In a paper published in February, 1872, I gave a synopsis of all the 

 North American fresh-water leeches then known to me. Since that 

 time, however, I have had opportunities to examine numerous living 

 specimens of most of the described species, and have been able to 

 study the variations more fully, and thus to improve many of the 

 descriptions. A few new species and marked varieties have also been 

 added to the list. I have also had opportunities to examine the various 

 collections of leeches obtained from the great lakes by Mr. J. W. Mil- 

 ner while engaged in the investigation of the fisheries ; by Mr. Oscar 

 Harger and Mr. T. M. Prudden while on the Yale scientific expeditions 

 to the Rocky Mountains in 1871 and 1873 ; by Dr. Josiah Curtis, Dr. 

 H. C. Yarrow, and H. W. Henshaw, while on the surveys west of 

 the one-hundredth meridian, under Lieutenant Wheeler; by Dr. Elliott 

 Cones, on the northwest-boundary commission ; and by Dr. Hayden's 

 expeditions. These collections have afforded many facts of great inter- 

 est in relation to the geographical distribution of the species, although 

 they have added but few new forms to those previously known. 



In order to facilitate the identification of the genera, the following 

 artificial key has been prepared. It is intended to apply only to the 

 genera included in the present paper. 



Analytical Jcey to the genera of American leeches described in the folloicing 



article. 



a. Head tapering, continuous with the body, (b :) 

 aa. Head dilated ; neck constricted, {h :) 



b. Ocelli marginal in a curved line; no proboscis, (c:) 



