353 
of Edinburgh , Session 1876 - 77 . 
sion, or intaglio , corresponding to the rounded relief, might be 
expected. The friend who had forwarded the fossil made diligent 
search, but found the upper part of the slab had been destroyed. 
In answer to a request for any animal tracks, or traces of organisms 
that might be met with in the same, or closely related deposits, he 
sent me several slabs of compact dark-coloured micaceous, sandy 
shale, on which some markings occurred. On splitting these, they 
were found crowded with tracks and casts of forms closely resem- 
bling that first sent, though the matrix is lithologically different. 
These shales intervene between bands of the so-called gritstone, in 
which the form now noticed was found. Its position was about 16 
feet from the surface, in a gritstone bed 4 feet thick, having above 
a deposit of alternating shales and gritstones nearly 15 feet thick 
and below a bed of limestone. 
The dark-coloured slabs present features of much interest. They 
contain traces of three species, the prevailing one being identical 
with that now before us, though none of the characteristic marks 
are so sharply outlined. They make it clear, however, that the 
surface on which the median line occurs is dorsal, and they estab- 
lish the bisulcate character of the track. On the reverse of one 
slab, the intaglio I was anxious to obtain shows distinctly that the 
dorsal line is ridged. This is also very well marked in another. 
In both the slight dorsal ridge is represented by a corresponding 
depression or furrow in the overlying shale. Even the outlines of 
these tracks are suggestive to the ichnologist. Some of them are 
comparatively deep, some shallow ; in some the bisulcate character 
is well seen, while in others the lower surface of the track is flat. 
The explanation of this is obvious. If, for example, we look at the 
tracks, say, of our common whelks ( Littorince ), we see that those 
formed in shallow shore-pools are flat ; those on very wet sand are 
slightly hollow, their edges presenting a corrugated appearance; 
while those made on fine sand beginning to dry, are marked at 
regular intervals with distinct transverse lines. It is not unusual 
to observe all these crossing one another, or seeming to form loops 
with each other. The waved whelk ( Buccinum undatem ) makes a 
bisulcate track on the sand when it is partially dry. Occasionally, 
however, it trails the sharp edge of its operculum in such a way as 
to give three sulci. Ichnologists may thus have the track of only 
