CASTLE: NORTH AMERICAN RHYNCHOBDELLID®. Bi 
The color pattern is somewhat similar to that of plana, but the contrasts are 
less striking and the colors less brilliant. The general color effect of the dorsal 
surface is a grayish brown. Marginal spots of light yellow are present, as in 
plana, on the non-sensory rings, but they are smaller and do not extend so far 
mesiad from the margin of the body. Practically all the larger papille appear 
as small white spots in a generally dark background. 
The median vitta is not a continuous light band as in plana, but is inter- 
rupted at regular intervals by spots of a darker color than the general dorsal 
surface. It begins as a narrow median light band on the head and neck, con- 
stricted or sometimes interrupted in the posterior part of somite v1., less often 
constricted or interrupted in somite v. also. About in annulus 19, somite rx., 
begins a narrow dark band which continues to the middle of somite xtr. 
Then come alternating light and dark spots, three of each. A light spot ex- 
tends over four annuli, a black spot over five as follows: Light spots, annuli 
29-32 (Figure 6), 38-41, 47-50; dark spots, annuli 33-37, 42-46, 51-55. An- 
other light spot covers rings 56-64 or 65, broadening out posteriorly so as to 
include the paired papille of somites xxii. and xxiv. (Figure C). This is fol- 
lowed by a median dark spot extending back past the anus to the margin of 
the posterior sucker. 
The posterior sucker is marked by alternating light and dark rays, very 
much as in plana (Figure 6); it also bears papille like those of the body 
farther forward. 
Ventrally the body is light gray in color, owing to the presence there of 
scattered pigment flecks, which, however, are not arranged in longitudinal 
bands as in plana. 
V. MUTUAL RELATIONSHIPS OF THE SPECIES 
DESCRIBED. 
The species described in this paper, with the exception of heteroclita, 
fall naturally into two distinct groups (Figure D, page 58), which may 
be designated respectively the stagnalis and the parasitica groups. The 
former includes the three species stagnalis, elongata, and fusca; the 
latter, parasitica and elegans, with the closely related European species, 
complanata and concolor. Heteroclita occupies a somewhat isolated 
position intermediate between these two groups. 
As arranged in Figure D., the species form a series in which there is 
from left to right an increasing degree of complexity of structure. This 
appears from an examination of rugosity, somite structure, crop diverti- 
cula, and certain other characters. 
In the species of the stagnalis group (1) there is a single pair of eyes 
derived from the sensillee of somite 111., (2) the genital pores are separated 
by a single ring, namely, the middle (sensory) ring of somite x11, and 
