POLYCH.'ETA. 



433 



cases that they really subdivide. It would appear that many of 

 the fossils of the Palaeozoic rocks which have been referred to the 

 Fucoids, under the generic titles Palceochorda, Palceophycus, &c, 

 may be really the filled-up burrows of wandering marine worms, 

 and such remains may at present be grouped together under the 

 common name of Planolites. 



While the above explanation will account sufficiently for many 

 of the objects which have been described under the name of Palceo- 

 chorda, and which are here spoken of under the title of Planolites, 

 there are many similar fossils for which Nathorst would account in 

 a different and a quite satisfactory manner. It has been shown, 



Fig. 348. — Planolites vulgaris, the rilled-up burrows of a marine worm. 

 Silurian (Clinton Group), Canada. (Original.) 



namely, by this well-known observer, that plaster-of-Paris casts of 

 the shallow grooves which worms make in crawling over the sur- 

 face of fine mud, present cylindrical vermiform markings precisely 

 similar in form and appearance to the fossils here spoken of under 

 the name of Planolites. Nathorst therefore concludes that most, 

 if not all, of such fossils are really convex casts of ivhat were origin- 

 ally grooves or furrows ; and he supports this contention by show- 

 ing that the worm-like markings in question are found to stand out 

 in demi-relief from the under surfaces of the strata. On this explana- 

 tion — which would probably account satisfactorily for very many of 

 these worm-like fossils — the structures here spoken of would really 

 be of the nature of tracks, rather than of filled-up burrows. At the 



