102 HULL SCIENTIFIC AND FIELD NATURALISTS’ CLUB. 
The dispersion of Planorbes is facilitated by their shape, 
which causes the smaller species to adhere readily to any 
object with which they come in contact. The small 
Pl. nautileus is abundant in ditches and ponds from Marfleet 
to Patrington, though a distinct gap in its range may be 
noted between Marfleet and Hedon, and it seems to be absent 
further inland. The salt ditch which runs parallel to the 
Humber prevents any voluntary migration to the ponds on 
the stray inside the Humber Bank, but it is found in the 
ponds near Marfleet, though absent from the one on Saltend 
Common. These ponds are a frequent resort of ducks and 
waders, and, if its dispersal was effected by birds, we ought 
to find nautileus in all of them. I am inclined to think that 
it has been introduced by cattle into the ponds on the Marfleet 
stray from the ditches in the district. The cattle are frequently 
transferred from the fields to the bank, and wice versa. There 
is no communication between the two strays; the cattle on 
Saltend are usually brought from a distance, or from fields 
towards Hedon and Preston, where nautileus does not occur, 
and there is no continual transference. 
On the Norfolk side of the Wash, Pl. nitidus takes the 
place of nawtileus, occurring in the ditches near the bank. I 
never found mautileus. In Holderness, it is just the reverse, 
nautileus being common and nitidus rare. The latter is found 
at Burstwick, in a short length of old ditch or pond by the 
roadside, 20 to 30 feet above the level of the neighbouring drain, 
—an interesting spot, of a type which is fast disappearing. 
There is no apparent drainage either in or out. Pl. nitidus 
is also found in a pond on the cliff top near Withernsea, and 
at Rise, both more than ten miles from Burstwick. The 
Withernsea pond may have been stocked by birds, though, ~ 
in that case, we might reasonably expect some resemblance ~ 
between the shells of the Humber and Wash areas, while the 
fact that one end of the Burstwick locality is a drinking place 
for cattle may explain its introduction there, though it must 
have been brought a considerable distance. In both cases, 
transport by water is quite out of the question. Their former 
distribution is instructive: both nétidus and nautileus are found 
in the lacustrine deposit at Skipsea, and the latter in that 
opposite Atwick (1892). They are absent from those at 
Ulrome, Atwick Gap, Mapleton, Neville’s Dyke, and 
Holmpton, and have not been taken in Hornsea Mere. 
Pl. spivorbis is numerous only in the marshy ground on 
Saltend and Marfleet Stray, just inside the bank, the former 
being one of the few instances of the occurrence of fresh-water 
