452 Mr. A. Hancock on Vei-mifoi'm Fossils. 



apparently originated, their walls, as We have seen, would not 

 entirely collapse, but the cylindrical form would be more or less 

 retained. It is therefore fair to suppose that the sedimentary 

 matter, as it was being deposited, would gradually find its way 

 into these lengthened tunnels or burrows after their submer- 

 gence, and ultimately fill them up ; but, the particles of such 

 infiltrated matter having a different arrangement from those 

 forming the general mass of the rock, the phenomenon presented 

 on breaking it up into slabs would necessarily occur : the casts 

 of the tracks would become isolated like the fossil remains of 

 any organic body, or might be left in relief on either the upper 

 or lower slab. 



The nodulous form, however, differs from the broad grooved 

 species in not keeping strictly to the same horizontal plane ; it 

 undulates slightly vertically as well as horizontally, so that the 

 bmTow is occasionally sunk entirely beneath the surface. In 

 such cases, as the tunnel cannot be formed by thrusting up the 

 surface, the animal must, as it presses forward, throw the exca- 

 vated matter backward, filling up the tunnel, either entirely or 

 partially, as fast as it is made. But here, too, as the particles 

 in the interior of the tunnel must have a different arrangement 

 from those that surround them, the cast of the track would be 

 liable to become isolated, on breaking up the rock, in the sauie 

 manner as if the burrow had been completely filled with infiltrated 

 matter. 



It is worthy of remark, that the nodulous forms, which have 

 neither median ridge nor groove, are rarely depressed, being 

 frequently cylindrical, and even sometimes deeper than wide. 

 This is just what might be expected, if the explanation now 

 given of these fossils be correct. At the junction of the nodules 

 there are septa formed (PI. XVII./, /), which pass for some 

 little distance inwards, and which may be looked upon as so 

 many transverse arches, giving support to the walls of the tunnel. 

 These must naturally assist in preventing their collapse, and 

 will in this way account not only for the cylindrical form of 

 these specimens, but also for their deficiency of median groove 

 or ridge. Their occasional greater or less depth below the sur- 

 face of the matrix must also have been favourable to the pre- 

 servation of their original form. 



The foregoing observations are entirely confined to the tracks 

 having a tunnel-form. We have seen, however, that there is 

 another kind which occasionally occurs on the slabs from North- 

 umberland. This is the first described species (PI. XVIII. 

 CyCjc), and is, as before stated, a simple furrow with a narrow 

 ridge on each side. It is ceitainly possible that this may have 

 been formed in the same manuer as the others, and that the 



