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both, of some living agent. Many of them are in positions 
where rain or wind cannot reach them, run almost vertically 
up into the rock, and are practically impervious to water at 
the highest point. (2) They are rarest on surfaces much ex- 
posed to prevailing winds, or where the rock approaches to 
a grit. (38) They usually occur on boulders, or projecting 
rocks, at no great distance from the surface of the soil; in 
not a few cases the turf had actually grown into them. 
(4) The axis of the burrow usually is not at right angles 
to the surface of the rock; often is only inclined at a slight 
angle to it, so that the burrow commences as a channel (if 
this be not a natural depression utilised), and sinks gradually 
into the rock. Frequently it is driven into some slight pro- 
minence, as though the burrowing animal had first sheltered 
itself under the lee of this, and then gradually worked its 
way deeper into the rock. (5) The burrows are very fre- 
quently curved. Sometimes the tangents to the axis at the two 
extremities, if produced to meet, would include an angle not 
very much greater than 90°. (6) Helices, especially H. ad- 
-spersa, are generally abundant in the neighbourhood of these 
burrows; empty shells are common in them, and in the 
freshest, smoothest, and most unweathered of them, he always 
found a living Helix. (7) The constriction in the upper part 
of the burrow characteristic of perfect Pholas excavations, is 
generally wanting, and though the burrow sometimes con- 
tracts towards the mouth, this is often not quite regular in 
form; so that a Helix that would exactly fit the end would 
be able to quit the burrow. At least he believes this to have 
been the case with all that he examined. The ends, also, of 
the burrows are, he thinks, generally rather flatter than is 
usual with Pholas holes. 
From the above considerations, and especially from the 
position of some cavities in the roof of a narrow crack in the rock, 
the conclusion is, he thinks, irresistible, that these are not the 
