FOOTPRINTS ON THE SANDS OF TIME 33 



surface of wet sand or inud, are found in the stratified rocks. 

 But, in the present state of knowledge, it is very difficult to 

 distinguish between those formed by worms and others made by 

 molluscs 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 Bolca (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 

 Palceontographica. ^ 



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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