28 ON CERTAIN VERMIFORM FOSSILS. 



burrow 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 track, in the same man- 

 ner 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 fre- 

 quently 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. VI. /,/), which pass for some little dis- 

 tance inwards, and which may be looked upon as so many trans- 

 verse 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 dejoth below the surface of the 

 matrix must also have been favourable to the preservation of 

 their original form. 



The foreoroino: observations are entirelv 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 second described species (PI. VII. c, c, c), 

 and is, as before stated, a simple furrow with a narrow ridge on 

 each side. It is certainly possible that this may have been 

 formed in the same manner as the others, and that the groove 

 may indicate the falling in of the roof of a tunnel ; but from its 

 great similarity to the track of Kroyera arenaria, it is more 

 probable that like it, it is a mere surface run formed by the ex- 

 posed animal ploughing its course. However this may be, its 

 nature cannot be questioned; it is undoubtedly a track, and so 



