34 
shell faster than their own bodies grow, consequently they 
desert the topmost whorls of their shells, which, becoming dry 
and brittle, break off. Untenanted houses—even those 
occupied by snails—soon fall into disrepair. The [most notable 
instance of this phenomenon is Bulimm decollatus —a species 
common in France and Spain. Of pond-snails, Limncea 
glabra, a species very common near York, becomes truncated 
from this cause. Other shells, chiefly freshwater shells, lose 
their top whorls by erosion, owing to the presence of carbonic 
acid in the water. Near York specimens of L. peregra are to 
be found decollated in ponds near Bootham Stray and else¬ 
where. I have, however, once found near York a pond full of 
our largest pond-snails, L. stagnalis, where the erosion had 
affected all parts of the shell equally, and so thin had they 
become that when I lifted some of them out of the water, the 
weight of the snail inside was sufficient to break the shell. 
When L. peregra lives in clear running streams, as e.g., at 
Guisborough, it is often decollated from mechanical rather than 
chemical action. 
L. peregra appears to attain its maximum of development 
in stagnant ponds and ditches. The spire is usually very short 
in these cases, and the mouth| of the shell immensely widened, 
the lip sometimes spreading to such an extent that it absolutely 
assumes another direction, and is considerably reflexed. In a 
ditch at Middlethorpe the specimens so closely approach in form 
another species (X. auricularia) that one of our best authorities 
was deceived at the first sight. Some specimens from tliis 
locality present the phenomenon of an addition to the lip after 
the shell had apparently assumed its normal proportions. One 
year almost all the specimens of L. stagnalis living in Seamer 
Mere, near Scarbro’, presented the same phenomenon, only in 
this case the added part starts from a point just inside the 
previously completed lip, and I can only suggest one explana¬ 
tion of this fact, viz., that some exceptional circumstances 
(possibly an unusually mild winter) had prolonged the life of 
the individuals beyond their allotted span. In other years 
I could never find a single specimen with the added lip, 
which in that particular year seemed almost universal. 
