W. Nicoll 
329 
to P. genu, but is not unreadily distinguishable from it by the difference 
in the distribution of the yolk-glands. 
The two species are about the same size but P. commune is broader 
and flatter than P. genu. The colour has a tinge of brown in it. The 
suckers have much the same size and relative position but the ventral 
sucker is distinctly more oval than in P. genu. In a 2 mm. specimen 
it measures 022 x 031 mm. The pharynx presents one of the dis¬ 
tinctive features of the species. It is almost fusiform in shape and 
much more elongated than in P. genu. Its size is 0T2 x 008 mm. 
The oesophagus is usually only about half the length of the pharynx. 
The diverticula extend to the posterior end. 
In respect of the genitalia the two species are practically identical. 
The genital aperture has the same situation and the cirrus-pouch the 
same extent. The latter is possibly a little more dilated at its posterior 
end. The testes and ovary are decidedly smaller than in P. genu (about 
frds the size). It is to the yolk-glands that we must look for the most 
distinctive feature of the species. Their extent and distribution are 
almost exactly the same as in P. genu, except that they are not 
interrupted at the ventral sucker. From the small amount of material 
at my disposal I am not able to confirm Odhner’s observation that this 
is a constant feature of difference between the two species, and in view 
of the amount of variation in the yolk-glands which is possible in other 
Allocrecidiinae, it is not to be denied that the two species might verge 
into each other. Taken, however, with the other less striking specific 
features, namely the elongated pharynx, the oval ventral sucker and 
the small ovary and testes, it forms a reliable enough distinguishing 
feature. 
According to Odhner the ova appear to be much narrower than in 
P. genu, but that is not the case in my specimens. 
Genus ii. Lebouria Nicoll 1909. 
Species 1. Lebouria varia n. sp. — {Lebouria) alac7'is (Lss.) Nicoll 1909. 
The doubt with which I regarded the identity of this form with 
Distomum alacre Lss. has now been resolved into a certainty that they 
are distinct. The differences are, however, not by any means striking 
owing to the rather considerable amount of variation which occurs. 
The species is almost exclusively confined to Gallionymus lyra in 
which it occurs fairly frequently, but rarely in number exceeding half 
a dozen. It appears to be distributed round the whole of the British 
