POLYCHiETA. 487 



very abundant in certain Silurian strata. In this fossil there is a 

 central narrow groove, which winds in serpentine bends over the 

 surface of the stone, and is supposed to represent the body of the 

 animal, bounded on each side by a broader and generally ill-defined 

 space, supposed to represent the foot-tubercles. That Crossopodia, 

 however, should be the petrified body of an Errant Annelide seems 

 almost incredible, and that it is even the track of one of these 

 creatures is extremely improbable. In well-preserved specimens of 

 any size, the impressions known under this name are seen to wind 

 backwards and forwards over the stone in a succession of long loops 

 which are placed quite close together, and which could hardly have 

 been produced by any animal in a movement of forward progression. 

 There is, indeed, some evidence that the impressions of Crossopodia 

 really cut directly across the laminae of deposition to some depth, 

 and that they have some direct, though at present not understood, 

 connection with Myrianites. 



As might have been expected, any fossils which can be supposed 

 with any probability to be the tracks of Annelides, or of other 

 marine animals, present themselves as depressed or concave markings 

 on the upper surfaces of the strata. The casts of these markings, 

 however, are often to be observed on the under surfaces of the beds, 

 and these, as a matter of course, present themselves as co?ivex or 

 elevated impressions. When the beds are vertical, or when the 

 specimens are not found actually in situ, it is impossible to distin- 

 guish between these two classes of specimens ; especially as some 

 elevated impressions, supposed to be tracks, do really occur on the 

 upper surfaces of the strata. Such impressions, in the opinion of 

 Principal Dawson, " have been left by denudation of the surround- 

 ing material, just as footprints on dry snow sometimes remain in 

 relief after the surrounding loose snow has been drifted away by 

 the wind, the portion consolidated by pressure being better able to 

 resist the denuding agency." 



As has been already pointed out, however, there is strong reason 

 for accepting the conclusion of Nathorst that many of the elevated 

 worm-like markings that are seen in muddy and sandy sediments are 

 in reality casts of grooved tracks or furrows which have been pro- 

 duced by the movements of Annelides and other marine animals 

 over fine silt. 



Before leaving this obscure subject finally, it may be well to 

 notice briefly one or two considerations which bear upon the ques- 

 tion of the origin and real nature of markings such as we have been 

 considering. In the first place, the late Mr Albany Hancock, in 

 an extremely able memoir, advocated the view that the vermiform 

 fossils of the older rocks may have been, in general at any rate, 

 produced by Crustaceans. He showed that similar markings are 



