ANNELIDA. 321 



shallow water. Markings of this nature are extremely abun- 

 dant in many of the older rocks, and in many cases no 

 doubt can be entertained as to their being really the tracks 

 of some marine animal. Even in these cases, however, it is 

 at present impossible, in the majority of instances, to dis- 

 criminate between the trails produced by Annelides and 

 those formed by Univalve Molluscs. There are, nevertheless, 

 certain tracks which w T e may fairly assert to be Annelidan. 

 This is especially true of the Silurian fossils upon which the 

 genera Ner cites (fig. 192, B) and Phyllodocites (fig. 192, A) 

 have been founded. In these cases we have long, sinuous, 

 and often sha.rply-bent impressions on the surfaces of the 

 strata, which consist of a central, broader or narrower axis, 

 representing the body of the worm, and of a series of lateral, 

 more or less leaf-like markings, representing the foot-tuber- 

 cles. These tracks, and others like them, have commonly 

 been supposed to represent the actual body of the Annelide, 

 now replaced by mud; but, as before remarked, it is very 

 difficult to conceive of such a replacement, and it is more 

 likely that we have simply the trail of the animal formed by 

 its serpentine wandering over the surface of soft mud. 



Another fossil, which is extremely abundant in the Silu- 

 rian rocks of some localities, and which has generally been 

 supposed to be the track of an Annelide, is Myrianites. In 

 ordinary specimens of this genus (fig. 192, c) all that is seen 

 is that the surfaces of the strata are marked by winding and 

 tortuous linear impressions, of extremely small comparative 

 width, and easily recognisable from the matrix by their 

 darker colour and slightly different texture. These mean- 

 dering markings wind over the surface of the stone in in- 

 definite undulations, often appearing to cross one another ; 

 and no one, looking at such a specimen, would be inclined 

 to doubt that he had to deal with the trails left upon the 

 mud of the sea-shore by some soft-bodied marine animals, 

 though he might question if these could be Annelides. Other 

 specimens, however, of the same fossil, which have been 

 carefully examined by the author, prove conclusively that, 

 in spite of appearances, Myrianites is not only not Annelidan 

 in its nature, but that it cannot possibly be the track of 



VOL. I. X 



