IQI4- Stelfox. — Mollusca of County Derry. 127 
this b}^ the fact that between the Bann and the central 
plain there is no real barrier to the migration of freshwater 
species, while to reach the present district it would be 
necessary for species to cross the Sperrin Mountains, or 
travel circuitously by the Bann or Foyle basin and then 
spread along the coast, which in the former case would, 
under present conditions, be impossible ; or be carried by 
chance means of dispersal from some habitat to the south. 
While touching on the subject of chance dispersal, I should 
hke to point out that if birds were responsible for the carry- 
ing of freshwater shells, the marshes of the Roe Estuary 
should yield a large list of species, since the shores of Lough 
Foyle have long been considered among the best districts 
in Ireland for the observation of migrating species. ^ 
A close study of the Irish distribution of the five 
central" species, which do occur in the present district, 
will show that they are among the most widely distributed 
members of the "central" group. In the neighbouring 
county of Antrim, three of them — Planorhis contortus, 
Physa fontinalis and Bithynia tentacidata — with Planorhis 
alhus and P. carinatus, alone represent the "central" 
species in the basin of the Bush ; and Physa fontinalis and 
Planorhis contortus occur on the island of Rathlin in the 
same county. 
LIST OF SPECIES. 
Umax maximus, Fer. — Near Limavarly, Roe Park, Walwortli Wood, and 
very common in the glens about Dnngiven. NNN.^ 
L. cinereo-niger, Wolf. — Recorded by Mr. Milne from Walworth Wood 
and without doul)t will be found in the glens of Banagher, near 
Dungiven. NNN. 
X. flavus, L. — Under slates in the yard of the hotel at Dungiven ; also 
in Roe Park (Milne), near the old mill. Probably introduced. ***. 
*I do not wish to deny that birds may carry freshwater shells from one 
pond to another close by ; but I do not believe that freshwater shells owe 
their present distribution to transportation by chance means of dispersal. 
2 The signs following the localities where each species has been found 
indicate the standing of the species in the district, according to Praeger's 
formula. See R. LI. Praeger : Report on Phanerogamia and Pteridophyta, 
Clare Lsland Survey. rroceedings, R. I. Acad., vol xxxi., part 10, p. 38., 
