Mr. A. Hancock oji Vermiform Fossils. 451 



each advance, — but that, instead of moving- in the same horizontal 

 plane, it alternately rose and sunk a little ; consequently a series 

 of nodules was produced, and the track acquired its peculiar 

 articulated appearance. This explanation is strengthened not a 

 little when we look at the side view of the cast of the track 

 (PI. XVII. fig. 2). The nodules are then distinctly observed to 

 be imbricated, and to pass, very much inclined, right through 

 the substance from top to bottom. 



We see in the ridged variety of the track of Sulcator arenarius 

 another approximation to the nodulous form ; at c, PI. XIV. 

 fig. 1, a few of the transverse ridges of this variety are repre- 

 sented, from which it will be seen that the articulated appear- 

 ance is very slight, and it is pretty obvious that it is produced 

 by the intermitting progress of the animal — the transverse 

 arched elevations or ridges undoubtedly indicate the successive 

 steps in the creature's path. These ridges, too, have much the 

 character of the transverse strise or wrinkles that cover the sur- 

 face of most of the larger fossils ; and it seems not altogether 

 impossible that the latter were produced in the same manner, 

 though they are much more closely set. It is more probable, 

 however, that these ridges in the fossil were caused by the 

 crumpling of the roof of the tunnel as the animal pressed for- 

 ward with a short, jerking motion. And here it must be borne 

 in mind that these transverse ridges, which in some individuals 

 are strong and well-defined, and even coarse and irregular, in 

 others are exceedingly delicate, and are occasionally entirely 

 wanting. This is usually the case on slabs dark with excess of 

 carbonaceous matter, indicating that the sedimentary material 

 of which the rock is composed was light and incoherent ; con- 

 sequently 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 track occasionally detaches itself 

 entirely from the matrix (PI. XVII. fig. 2). Mr. Wood informs 

 me that a blow with a hammer is very liable to separate the 

 specimen from the rock, leaving a cast on both the upper and 

 lower slab ; and in his first paper he remarks, " If the appear- 

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

 circular form on both the upper and lower surfaces V 



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 arenarius 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 



