382 C. O. WHITMAN. 
Habitat.—Found in the neighbourhood of Cambridge; 
geographical limits unknown. 
Abbreviated Somites.—There are twenty-six somites, of 
which the first six and the last four are abbreviated. The 
abbreviation at the anterior end agrees with what has been 
seen in Hirudo. The 24th, 25th, and 26th somites have each 
two annuli or remnants of annuli, and in so far agree with the 
Hirudo type. Inthe 23rd somite we find an important difference 
between the two genera; for in this somite there are at least 
four annuli in Macrobdella against three in Hirudo. The 
second annulus of this somite (95th in fig. 59) must be regarded 
as two annuli in process of consolidation; as its two halves 
show a well-marked separation at one (left in the fig.) and 
sometimes both margins. The two halves are, together, only 
a trifle thicker than the 94th annulus; but they are much 
thicker than the 96th. This peculiar double annulus is found 
in several (perhaps all) other species of Macrobdella. The 
process of abbreviation has only fairly begun in this somite, 
and has just reached a point that leaves it doubtful whether we 
have four or five annuli. 
Differential Characters.—The genus Macrobdella is 
distinguished from Hirudo by the following characters : 
1. Copulatory glands. 
2. Four (or five) annuli in the 23rd somite. 
3. Neither the buccal nor the post-buccal annuli are united 
on the ventral side. 
4. Cephalic lobe smaller. 
It remains to be seen whether this genus may be sub- 
divided according to the number of rings separating the 
sexual orifices. 
LeptostoMa PIGRUM, g. et sp. nov. Pl. XVIII, figs. 21—27. 
Diagnostic Characters. 
Body large and fleshy, tapering towards the head more 
rapidly than in Hirudo (figs. 22 and 23). 
Length of one of the larger specimens, swimming, 16°5cm. ; in extension, 21cm. 
Width ~ es = 2 cm.; at rest, 2°5cm. 
