POOTPRINTS ON THE SANDS OF TIME 33 
surface of wet sand or mud, are found in the stratitied rocks. 
But, in the present state of knowledge, it is very difficult to 
distinguish between those formed by worms and others made by 
molluses or even crustaceans. The fossil known as Nereites, from 
Silurian slates at Wurtzbach, is probably the track of a worm. 
On the other hand, some of the tracks attributed to worms may 
have been really made by gastropod molluscs, such as whelks. 
One fossil track, known as Crossopodia, resembles the track made 
by a living Purpura lapillus, a well-known sea-shell. It is only 
in those strata which are very favourable to the preservation of 
organic remains that we can expect to find any trace or impres- 
sion of the actual body of such a frail and perishable thing as a 
worm; but, incredible as it may seem, fossils of this nature 
occur in that most wonderful formation—the famous Solenhofen 
limestone, in which so many valuable treasures have been 
found; also in the Eocene slates of Monte Bolea (Italy). In 
these rare cases the form of the worm’s body is actually seen, 
and the fossilised jaws occur in their natural position. Ex- 
amples of these interesting specimens are beautifully rendered 
by chromolithography in the German monumental work 
Paleontographica. 
Several geologists, such as Poulett-Scrope, Strickland, Buck- 
land, Salter, and others, have published the results of careful 
comparisons of tracks made by living animals on the sands of the 
seashore, or on flat surfaces of mud left exposed by the drying up 
of a pond, or by other causes, but have not given drawings of the 
recent markings on which their conclusions are based. Professor 
Emmons and Professor T. McKenny Hughes, however, have 
figured some recent tracks in illustration of fossil ones. The 
former geologist came to the conclusion that certain imprints 
upon some very old rocks—the Taconic Shales of Maine and New 
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