Mr. A. Hancock on Vermiform Fossils, 449 



It is this last form, particularly the smaller and more compli- 

 cated variety, that so closely resembles the track of Sulcator 

 arenarius (PI. XIV. fig. 1). The folds or windings are precisely 

 similar, and so is the median groove. It differs chiefly in being 

 occasionally much larger, and in rising up more boldly in relief 

 from the matrix, though in these respects they sometimes closely 

 approximate. Dimensions, however, can be of very little conse- 

 quence ; for of course the larger the animal, the larger the track. 

 The relief may also be influenced by other conditions ; the qua- 

 lity of the substance in which the tracks are made must likewise 

 be taken into account. 



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

 diately 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 cir- 

 cumstances, 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 Car- 

 boniferous rocks which have revealed to us these curious vermi- 

 form 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 

 Howes, low down in the middle group of the Yoredale series, 

 and called by him the flagstone beds of Howes." And, in 

 speaking of the nature of these flagstones, Professor Phillips 

 states, in his work on the 'Mountain Limestone District of 

 Yorkshire/ that they are formed of " a laminated rock, composed 

 of small worn grains of quartz, mica with or without felspar, 



