484 ANNELIDA. 



same time, there are undoubtedly some examples which may be 

 regarded as burrows and not as tracks, since there are specimens in 

 which the cylindrical worm-like markings pass obliquely from one 

 lamina to another, instead of being confined to a single surface of 

 deposition. Another point that may be noted in connection with 

 this subject is, that it has been shown by Nathorst that the tracks 

 of certain Annelides upon the surface of mud or sand are branched. 

 The fact, therefore, that many of the fossils described under such 

 titles as Palceochorda and Palaophycus are seen to branch, would not 

 be in itself sufficient to preclude acceptance of the explanation of 

 their origin put forward by Nathorst. 



In connection with the burrows of worms, a word may be said as 

 to the curious vermiform bodies which occur abundantly in the 

 Lithographic Slates (Jurassic) of Solenhofen, and which have been 

 described under the name of Lumbricaria. The fossils in question 

 are about as thick as a quill in some cases, but in others are only 

 of the dimensions of twine, and they form convoluted and confused 

 elevations on the surface of the beds. They have generally been 

 looked upon as of the nature of casts of the alimentary canal — true 

 " worm-casts " in fact — of marine worms j and, judging from their 

 appearance, this explanation of their nature is probably correct. 



III. Trails and Tracks. — Lastly, we have to deal briefly with a 

 great group of fossils which have been supposed to be of the nature 

 of the " trails " of Errant Annelides — that is to say, markings formed 

 by the animal dragging its soft body over the surface of wet sand or 

 mud, between tide-marks or in shallow water. Markings of this 

 nature are extremely abundant 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 discriminate 

 between the trails produced by Annelides and those formed by 

 Univalve Molluscs or by Crustaceans. There are, nevertheless, 

 certain tracks which may be regarded with considerable probability 

 as Annelidan. This is especially true of the Silurian fossils upon 

 which the genera Nereites (fig. 349, b) 2X& Phyllodocites (fig. 349, a) 

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

 often sharply-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 mark- 

 ings, representing the foot-tubercles. 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. 



