IIIBEKNACULA. 
?)10 
/)n;/sseu!ii<( easts oil’ its l)yssus aiiil also retires beueatli the bottom 
mud, as its temperature is always higher than that of the super- 
imi)osed water. 
'J'he amount of cold mollusks can endure is, however, not unlimited, 
as iMopuiu-Tandou records that Helices, though capable of enduring 
1()° Fahr., freeze at 14° Fahr., and the endurance of this great cold 
implies the ])ossession of internal heat sufficient to counteract the 
reduction of temperature, perhai)s i)artially due to the excess of 
respiratory ])igments present in the tissues in winter. 
IIiiimiNACULA {lill>eni((culnm, a winter abode) or winter shelters may 
lie more or less adventitious or may be places of more regular resort 
utilized liy the mollusks, not only during their seasons of protracted 
torpidity, w hether arising from cold or dryness, hut as places of safety 
during their diurnal siesta. 
In certain districts the carhoniferous limestone rocks are almost 
honeycomhed hy tubular e.xcavations, which, though iirohably made 
and (diietly tenanted by //c//,r ((.'tjH'/ya, are occasionally occupied by 
other species, not only to pa.ss through the periods of hihernation 
and a'stivation, but as regidar resorts for rest and shelter. 
d’hese galleries, which almost invariably take an upward direction, 
perpendicular to the bedding of the strata, rarely occur on rocks 
apjiroacbing grit or on rock-taces exposed to the prevailing winds, hut 
are usually clu.stered heneath iirojecting rock-ledges or pierced in 
the face oftho.se clilfs facing ea.st or north-east, heing thus protected 
from the wet west and south-west winds. When placed uiion the 
i'ace of a vertical cliff the buiTows may originate as a slight channel 
and sink gradually into the rock ; frepuently, however, tliey originate 
beneath a slight prominence, as though the snails had fjrmerly 
sheltered there and gradually worked their way into the stone. 
These rock tunnels are generally about (jiie inch in diameter and 
sometimes three inches or more in deiith, smooth and regularly 
slnqied inside, but often containing subsidiary depressions upon their 
walls, due to a persistent n.se of those iiarticular sjiots as resting jilaces 
by many .snails ; they must not, however, he confused with the oval 
or circular cavities due to weathering nor with the Ihtter.spar con- 
cavities so common in the magnesian limestone, whose w'alls are often 
encrusted with crystals of lime, as such natural cavities freipiently are. 
These tunnels were formerly surmised to be the work of the Pholades 
or other marine boring mollusks, during the period the rocks were 
