324 ANNULOSA. 



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 convex or elevated impressions. 

 When the beds are vertical, or when the specimens are not 

 found actually in situ, it is impossible to distinguish between 

 these two classes of specimens ; especially as some elevated 

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

 iipper surfaces of the strata. Such impressions, in the 

 opinion of Principal Dawson, " have been left by denudation 

 of the surrounding 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." 



Before leaving this obscure subject finally, it may be 

 well to notice briefly one or two considerations which bear 

 upon the question of the yrigin 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 pro- 

 duced, at the present day, by small Amphipod Crustaceans 

 (Sulcator arenarius and Kroyera arenaria), which burrow 

 immediately below the sand on the sea-shore, and give rise 

 to the following appearances : (1.) Large tracks, about 

 3-8ths of an inch wide, slightly raised, ribbon -like in 



