190 Correspondence. 
to produce the effect. It was not long before I discovered that these 
depressions were confined to a certain well-defined zone in the sandy 
shore, none appearing above or below it; further, that they were 
the burrows of the Gammarus, or Sand-hopper. Each of these 
conical depressions contained water; and as the wind was blowing 
the sand from above, a cusp was produced round each towards the 
leeward side of the minute crater. (See Woodcut.) Now, all that. 
was necessary to preserve these Gammarus-burrows, was simply 
their being filled up by the dry sand, and thus kept from the de- 
grading action of the advancing tide. 
TALITRUS (Gammarus) SALTATOR, Edw. 
The common Sand-hopper in its burrow. 
(a, a, w), apertures of three burrows upon the surface of dry sandy beach (d, d);—(6) burrow 
seen in section around which the sand is always moist ;—(m, m) depth at which the beach is 
moist at low water. The arrows indicate the direction of the wind. 
Another consideration satisfied me that in no instance can we 
expect to find the fossil marks of rain-drops unless a special condi- 
tion, such as Gideon of old prayed for, were obtained; namely, that 
one portion should be dry, and allaround moist. For all the casts of 
foot-prints, gasteropod-tracks, and the so-called fossil rain-drops, one 
condition is essential,—that before the next tide these markings must 
be filled up with dry dust or sand. It is therefore clear that no such 
traces of rain-drops can be found, as all around must have been wet, 
and the next tide would certainly obliterate any trace of the depres- 
sions. Great caution is necessary to distinguish the track of a Gas- 
teropod from that of a Nereid on the soft sand. When the track of 
a whelk is being filled up by the dry sand blown into the depression 
in the line of progress, no difficulty is felt in recognizing it as the 
track of a Gasteropod ; but should the wind blow at right angles to 
the track of the Mollusc, a series of sete-like markings will be 
observed to leeward, caused by the dry sand adhering to the moist. 
In this instance a geologist would naturally assign the markings to 
the impression of Graptolites pristis or G. sagittarius. And if the 
wind suddenly shifted to the opposite direction, another series of sete 
