368 Cc. O. WHITMAN. 
because they were said to have fewer teeth than are usually 
found in Hirudo medicinalis. It is evident too that certain 
aquatic Leeches, although much further removed from He- 
mopis than this genus is from Hirudo, have, nevertheless, been 
associated with the former on the same insufficient ground. A 
similar blunder has been made in the attempt to make the 
entire absence of denticles a basis of generic association. The 
discovery of toothless Leeches in different parts of the earth, 
which have evidently descended from different species of denti- 
culated Leeches, shows how unreliable and worthless genera 
are when founded on such characters. But if the entire absence ~ 
of denticles is no certain indication of generic affinity, how 
much less certain is a difference in number only. I have satis- 
fied myself that two Leeches belonging to two distinct genera 
may often agree more nearly in the number of denticles than 
two species of the same genus. Numerous instances of this 
kind are at hand, but one or two will be sufficient here. No 
one will deny that Macrobdella, Verrill, and Hirudo are quite 
distinct genera. Now Leidy! has described a species of Ma- 
crobdella with fifty-five teeth; and Schmarda? states that 
Hirudo quinquestriata (from Australia) has from forty- 
eight to fifty teeth. It is also stated by Schmarda that H. 
multistriata (Ceylon) has about one hundred teeth. 
Again, Macrobdella floridana Verrill, has only “ about 
twenty acute teeth,”’ thirty less than Macrobdella, Leidy. In 
M. sestertia (n.sp.) I have found one jaw furnished with thirty- 
nine, the second with forty-three, and the third with forty-six 
teeth. Schmarda found only thirty teeth in Hemopis cey- 
lonica, which is a Land Leech belonging to Hemadipsa—a 
genus sufficiently distinct from Macrobdella. 
If we are to avoid increasing the number of genera until 
they equal or nearly equal the number of species, it is evident 
1 Leidy, ‘Proc. Phil. Acad. Nat. Sci.,’ p. 230, 1868. 
2 Schmarda, ‘ Neue wirbellose Thiere,’ i, 2nd part, p. 2, 1861. 
3 Verrill, ‘Synopsis of the North American Fresh-water Leeches,” p. 669, 
1874. 
