26 ON CERTAIN VERMIFORM FOSSILS. 



however, that a variety of the track of Kroyera arenaria has the 

 ridge of sand thrown up broken into nodules, giving to it a 

 beaded character. This on a small scale has a considerable re- 

 semblance to these nodulous forms. But a much nearer approxi- 

 mation is found in the nodulous tracks of Sulcator arenarius, 

 particularly of the one made in confinement (PL' III. fig. 2, h). 

 Had the animal continued to form its track in this manner, the 

 resemblance to those on the slabs would have been almost com- 

 plete. It may, therefore, be assumed that the animal which 

 made those nodulous tracks, like our small crustaceans, pushed 

 along in its path step by step, resting awhile after each advance, 

 but that, instead of moving in the same horizontal plane, it al- 

 ternately 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 (PL VI. 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, PL III. fig. 1, 

 a few of the transverse ridges of this variety are represented, 

 from which it will be seen that the articulated appearance is 

 very slight ; and it is pretty obvious that it is produced by the 

 intermitting progress of the animal — the transverse, arched ele- 

 vations, or ridges, undoubtedly indicate the successive steps in 

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

 of the transverse strias or wrinkles that cover the surface 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 crumpliug of the roof of 

 the tunnel as the animal pressed forward with a short, jerking 

 motion. And here it must be borne in mind that these trans- 

 verse ridges, which in some individuals are strong and well de- 

 fined, and even coarse and irregular, in others are exceedingly 



