24 ON CERTAIN VERMIFORM FOSSILS. 



track. The relief may also be influenced by other conditions; 

 the quality of the substance in which the tracks are made must 

 likewise be taken into account. 



The crustacean, as we have seen, forces itself onward immedi- 

 ately beneath the surface of the sand, which is thrust up by its 

 back, and as it moves along a sort of arched tunnel is thus 

 formed ; but as the sand is incoherent — unmixed as it is with 

 any material that could give it consistency — the roof falls in 

 immediately the animal ceases to give it support, and ultimately 

 the relief of the track is very small. As the arch falls, it must 

 either break along the centre or thrust out the sides ; the latter 

 is impossible, hence the median groove. Had the beach been 

 composed of sand with a large admixture of argillaceous matter 

 or tenacious mud, it is very obvious that the tunnel would have 

 had a greater tendency to retain its original form; and that had 

 it been submerged before it had subsided to any great extent, 

 such an infiltration of matter might have taken place as to pre- 

 vent any very extensive collapse. The roof under such circum- 

 stances would split along the centre, and the margins of the 

 fracture would either fall inwards and form a groove, or be 

 pressed outwards and become a ridge. It is also possible to 

 conceive that the substance composing the shore might be so 

 tenacious that the roof of the tunnel would scarcely subside at 

 all, and that consequently there would be no fracture along it, 

 and therefore neither groove nor ridge. 



Now, these cases, which are hypothetical so far as they con- 

 cern our crustacean tracks, do not appear to be so in regard to 

 the conditions that prevailed during the deposition of the carbo- 

 niferous rocks, which have revealed to us these curious vermiform 

 fossils. The rocks from which the Yorkshire specimens were 

 procured are, Mr. Wood says, " apparently equivalent to the 

 flagstone beds placed by Phillips in his section of the hills about 

 Hawes, low down in the middle group of the Yoredale series, and 

 called by him the flagstone beds of Hawes." And in speak- 

 ing of the nature of these flagstones, Professor Phillips states, in 

 his work on the " Mountain Limestone District of Yorkshire," 



