ON CERTAIN VERMIFORM FOSSILS. 25 



that they are formed of " a laminated rock, composed of small, 

 worn grains of quartz, mica, with or without felspar, 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 composed of a finely laminated, 

 shaly, compact, close grained, micaceous sandstone, contain 

 argillaceous matter. The slabs from Northumberland have ex- 

 actly the same composition, and apparently 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 crusta- 

 cean 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 material 

 at once explains the higher relief of the fossil, the occasional 

 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 tunnelled 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 characterised, as we 

 have seen, by a remarkable nodulous or articulated appearance (PI. 

 V. a, «), which has been supposed to indicate the presence 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, 



VOL. IV. PT. I. D 



