450 Mr. A. Hancock on Vermiform Fossils. 



and other minerals, occasionally calcareous, carbonaceous, and 

 argillaceous : the mica or carbon, lying in particular planes, 

 causes the minute fissility of the stone ; and bands of mica or 

 argillaceous matter separate it into thin flags or beds. The 

 tops and bottoms of gritstone rocks are often thus laminated ; 

 plates becoming very sandy change to flagstone; grit rocks 

 becoming very argillaceous assume the same character." It 

 would thus appear that these fossiliferous slabs, which are com- 

 posed of a finely-laminated, shaly, compact, close-grained, mica- 

 ceous sandstone, contain argillaceous matter. The slabs from 

 Northumberland have exactly the same composition, and appa- 

 rently belong to the same series of rocks. It is fair, then, to 

 assume that the matter composing these flagstones was originally 

 of a more tenacious consistency than the sand of our shores, and 

 therefore, were the tracks found on the slabs made by even the 

 very same crustacean that occurs on our coast, some differences 

 might be expected to exist between such tracks and those formed 

 on the beach. 



Such differences, we have seen, are very slight, and may all 

 be accounted for in this way. The greater tenacity of the ma- 

 terial at once explains the higher relief of the fossil, the occa- 

 sional substitution of a ridge for the median groove, and the 

 deficiency of either groove or ridge in those whose relief is so 

 excessive as to have become almost cylindrical. A similar smooth 

 variety, as previously stated, sometimes occurs on the beach ; 

 but this arises from the fact of the crustacean having tunneled 

 deeper than usual, and in this case the relief is not great. 

 Smooth tracks are also found on the slabs, arising, probably, 

 from the like cause. 



The specimens described by Mr. Wood are characterized, as 

 we have seen, by a remarkable nodulous or articulated appearance 

 (PI. XVI. a, a) } which has been supposed to indicate the pre- 

 sence of rings of some Annelide ; and, indeed, were it not for 

 the light derived from the crustacean tracks on our shores, it 

 would not be easy to conjecture a more plausible explanation. 

 We have seen, however, that a variety of the track of Kroyera 

 armaria has the ridge of sand thrown up broken into nodules, 

 giving to it a beaded character. This, on a small scale, has a 

 considerable resemblance to these nodulous forms. But a much 

 nearer approximation is found in the nodulous tracks of Sulcator 

 arenarius, particularly of the one made in confinement (PI. XIV. 

 fig. 2 b). Had the animal continued to form its track in this 

 manner, the resemblance to those on the slabs would have been 

 almost complete. It may therefore be assumed that the ani- 

 mal which made those nodulous tracks, like our small crusta- 

 ceans, pushed along in its path step by step, resting awhile after 



