The Hirudinea of Illinois. 509 



General DescrijJtion. — This fine leech is so well known and 

 has been so fully described by Say ('24), Leidy ('68), Verrill 

 ('74), and Brooks ('82), that only a few notes on certain 

 features need to be added. 



The anterior sucker is provided with a rather wide unseg- 

 mented and very mobile border which materially increases 

 its capacity for expansion. Anteriorly a distinct median 

 emargination corresponds with a deep ventral sulcus, which 

 divides the upper lip and is flanked by a pair of scarcely less 

 deep sulci. When strongly contracted the upper lip is folded 

 and turned into the buccal chamber, where it is almost 

 entirely concealed by the lateral lobes formed by the margins 

 of somite IY. 



A well-marked clitellum is seldom present. In one example 

 it is firm and thick and extends over eighteen annuli, from 

 the posterior half of X b 5 to XIV b 2. In the fully retracted 

 state the male pore appears as a rather large opening on the 

 furrow XI/XII, into which the surrounding rugosites are 

 converged and inflected, forming a small sinus perhaps com- 

 parable with the pit of Philobdella. These inflected parts 

 may be everted, in which event they form a prominent conical 

 organ having deeply fluted sides and the small male aperture 

 at the apex. As protrusion of the parts takes place annulus 

 XII b\ becomes relatively longer at the middle and comes 

 to support almost the entire base of the papilla, so that the 

 male pore now lies well within the boundaries of this annulus. 

 I have never seen any other penial organ protruded. 



When fully developed each of the four copulatory gland pores 

 lies in the center of a prominent rugous area, the four being 

 arranged in the form of a square. The anterior pair extends 

 half over annulus XIII b6 and equally over XIV bl, and 

 the posterior pair holds a similar relation to XIV b 1 and £2. 

 The pores consequently open on the line of the furrows 

 separating these annuli. 



Annulation. — The external annulation differs but slightly 

 from the closely allied species M. sestertia, figured by Whitman 

 ('86). The principal differences (characters stated as they 

 occur in M. decora) are as follows : 1. The annulus IV (a 1 -+- 



