TERRESTRIAL AND FRESHWATER ANIMALS. 181 
vegetation, caught in the embayed water in each trough, and I have 
thought how easily it might happen that if a track were so filled, 
covered, and pressed down, the relief imprint might have a thin film 
of carbonaceous matter over it. 
Again we observe from the different rate of progress of animals 
with legs or swimming-apparatus, as they get into the thicker or 
thinner parts of the mud, that no inference can be drawn as to the 
form of the animal, or distance between its legs from the strokes it 
leaves at right angles to, or oblique to the direction of its motion. 
The oars of a boat that has run aground return to the same place 
at each stroke. When the boat begins to move, the distance 
between the lines where the oars are lifted out of the water 
increases, until at last the boat may glide on and the distance 
between the dip of the oars may be longer than the stroke of the 
oar through the water. So, if a cock <chafer has fallen on a some- 
what thick part of the slush, many strokes of his legs advance him 
but a little way, and the ridges oi mud pushed up by each stroke 
are very close together. They may be nearly at right angles to 
the direction of motion, or the animal may draw them very close to 
his body and never get a good sweep. This commonly happens 
when the slime is thick, and in such a case the strie are only 
slightly inclined to the direction of motion. 
I have seen such a track made under very favourable circum- 
stances. The insect had fallen, from an overhanging tree, into 
water from which it struggled towards shore. Wines T saw it, it 
had just reached mud of sufficient consistency to allow it to make 
some progress. ‘The result was the formation of a fringed Bilobite. 
The ovipositor, dragged through the mud, formed the Saeitan line, 
the stroke of the legs produced the ridges on the lobes. There 
were two less distinct marginal furrows formed, as it appeared to me, 
either by the ends of the wing-cases (elytra), or by the bristly edge 
of the animal’s body ; I could not observe this point clearly ; these, 
somewhat interrupting the ridges due to the legs, gave the appearance 
of a fringed edge. Unfortunately, I was unable to preserve this 
track, and attempts to reproduce exactly the conditions were un- 
successful. Of all the tracks of land and freshwater animals which 
I haye seen, this was the one which most closely resembled Cruziana 
semiplicata. % 
Water-beetles (Dytiscus &c.), however, are frequently caught in 
the “ slurry ” and produce tracks somewhat similar, one of which is 
figured (Pl. VIII. fig. 1, 1 @), in which C D is the continuation of 
meee cand. (Pl. Xd, ‘fic. 3) in which a median line can be more 
clearly traced *. 
These differ from the tracks made te land-beetles in several im- 
portant points; for the water-beetles have long rowing-legs, which 
sweep far beyond the short walking-legs, which therefore with the 
trail of the body produce a median lobe, the long rowing-legs 
producing two similar marginal lobes. When the animal gets on 
* The figures have been drawn for me by Mrs. Hugh Strickland, a skilful 
artist, who helped her father to illustrate the Ichnology of Annandale. 
