ON CERTAIN VERMIFORM FOSSILS. 27 



delicate, and are occasionally entirely wanting. This is usually 

 tlie case on slabs dark with excess of carbonaceous matter, indi- 

 cating that the sedimentary material of which the rock is com- 

 posed was light and incoherent ; consequently, the specimens on 

 such slabs are not only devoid of those peculiar ridges, but are 

 also in very low relief, some being quite as little elevated as the 

 tracks on the sea-beach. 



The cast of the nodulous tracks occasionally detaches itself 

 entirely from the matrix (PL IV. fig. 2). Mr. Wood informs me 

 that a blow with a hammer is very liable to separate the speci- 

 men from the rock, leaving a cast on both the upper and lower 

 slab ; and in his first paper he remarks, " If the appearances 

 above spoken of are but markings, how could they show a cir- 

 cular form on both the upper and lower surfaces?" 



This at first sight seems a formidable difficulty, and were the 

 tracks such mere superficial markings as is there supposed, this 

 objection would be fatal to the view now taken with respect to 

 the nature of these fossils. It has been shown, however, that 

 the track of Sulcator arenarkis is a tunnel, and with the aid of 

 this fact the difficulty at once disappears. If the tunnel-tracks 

 were formed in a tenacious material, such as that from which 

 these slabs have 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 de'posited, would gradu- 

 ally find its way into these lengthened tunnels or burrows after 

 their submergence, and ultimately fill them up ; but the par- 

 ticles of such infiltrated matter, having a different arrangement 

 from those forming the general mass of the rock, the phenome- 

 non 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 



