280 MR. C. REID ON THE NATURAL HI8TORY OF T80LATED PONDS. 
though not belonging strictly to the subject of this paper, one 
occurrence of this species is so suggestive of recent transport that 
it may be placed on record. Hornsea Mere, in Holderness, is 
a natural lake or broad of considerable size, with an outlet to the 
sea, but with no connection with other basins. In this Mere live 
numerous fresh-water mollusca, but though the species have been 
recorded more than once, Limncea glabra does not appear in the 
list. On one occasion I found, however, a single full-grown dead 
specimen on the shore of the lake, but repeated search did not 
produce a second. This occurrence of a single specimen of 
a gregarious genus like Limncea suggests that here was a case in 
which only one young individual was transported to the lake, and 
that the mollusc, therefore, grew up and died without leaving 
descendants. 
The genus Planorbis is next in abundance in isolated ponds ; 
in fact, the species spirorbis and vortex rank next to Limncea 
peregra in number of occurrences. Planorbis albus, P. nautilevs, 
P. complanatus, P. carinatus, and P. contortus have each been 
seen once or twice, P. corneus only once. The only species of the 
genus noticed in dew-ponds on the Downs are P. spirorbis and 
P. vortex ; these hat shells tend to cling to a stick or piece of weed 
taken out of the water, the other species generally drop off. 
In the genus Physa, one record of P. fontinalis , and one of 
P. hypnorum is all that have been made. A vigorous colony of the 
last-named species was observed in Holderness in a small pool in 
a ditch, with no outlet except through a pipe, which formed 
a raised spout discharging into another ditch. The lower ditch did 
not contain the species, and I did not observe it anywhere in the 
neighbourhood. Ancylus lacustris has only been noticed once in 
an isolated pond. A. fluviatilis is still unrecorded. 
Turning next to the Bivalves, one is surprised to find that no 
species is at all common in isolated ponds, though they are said 
often to be transported attached by the closing of their valves to 
the toes of birds. Anudun and Unio have not been observed, 
Sphcerium corneum has occurred once, Pisidium amnicum once, 
P. pusillum and P. henslounanum several times ; but it is possible 
that the small Pisidia may often be overlooked. 
The list of plants found associated with the above-mentioned 
mollusca contains a few only of our aquatic species ; but nearly all 
