TERRESTRIAL AND FRESHWATER ANIMALS. 183 
always have a small rim all round the hole, when there has been a 
straight downpour ; on one side, away from the direction in which 
the rain or hail was coming, when that has fallen obliquely—see 
Piel, fig. 7. 
Secondly, besides this there is the round hole left by bursting 
bubbles—see Pl. XI. fig. 6. These are due to the caving in of the 
_ mud when the bubble has burst and left a cavity; and they there- 
fore have not the rim due to the violently displaced contents of 
the depression, as in the case of the rain-spots. 
Thirdly, there is the hole made by worms, burrowing into or 
coming out from the mud. These are often in pairs, as they 
showed only a short time above ground or remained only a short 
time below. In this case, as might naturally be expected, the 
margin of the hole down which the worm went shows the indrag 
of the animal’s body in the rounding off of the edge, and the hole 
from which it emerged shows the out-push in a kind of rim. 
The holes soon get filled with mud. In these we often have Arenz- 
colites didymus exactly reproduced. Sometimes a worm or larva, 
or other burrowing animal, bores just under the surface of the mud 
so close to the surface that it raises a round archway in considerable 
relief above the level of the surrounding mud, which is seen in 
section as the roof of the tube or tunnel along which the animal 
has passed (Pl. X. fig. 3). Sometimes a worm pushes the fore 
part of the body out along the surface, making trials in several 
directions and producing a long cylindrical tube with a flattened 
branched extremity (see Pl. XI. fig. 4). As the posterior part of 
the animal is not dragged after the anterior in this case, the barred 
track is not obliterated. The cross markings are, however, shown 
a little too strongly in the figure. This is quite unlike the forms 
described and figured by Nathorst (op. cit. pl. i. fig. 5, pl. v. fig. 3, 
&e.). Some of his are curiously like Oldhamia radiata. The organic 
origin of this fossil is more obvious in the other species, O. antiqua. 
Some smaller animal (? a beetle) pushes its head out at very 
regular intervals, forming a long hollow tube with symmetrically 
arranged cell-like openings, just such as might be referred to a 
zoophyte with ccenosare and hydrothece (Pl. XI. figs. 1 & 2). 
Should the mud in such a case harden sufficiently to allow of 
the infilling of the tube, we should have a track in relief on the 
upper surface of a slab, and showing a solid section and branched 
extremity (see Pl. XI. fig. 4), or a branching base and long trailing 
extremity (Pl. XI. figs. 1 & 2). 
The tracks which somewhat resemble Nereites and Myrianites 
are produced by a great variety of animals. I have seen Limnca 
peregra make a mark on very fine wet mud in a ditch, exceedingly 
like Nereites ; but this was quite exceptional, as the animal generally 
glides along smoothly, and it seemed that the interrupted motion 
in the case observed, was due to the necessity of pushing the head 
far forward and lifting the shell with a jerk. Very smali frogs 
crawling out of the slush, where hopping is impossible, produce by 
their alternate step a lobed mark analogous to Nereites;: while 
