TRACKS, TRAILS AND BURROWS 1091 



covered by sediments and had not yet cooled completely. This is 

 not necessary, however, since, as shown in an earlier chapter, foot- 

 prints may be preserved for a long period by mere drying of the 

 mud. Even the delicate impressions of the web membranes of 

 the foot were frequently preserved, which seems to indicate that 

 the arenaceous mud must have been fairly hard and resistant 

 before the next layer of sand was spread over it. This later layer 

 on its under side furnishes accurate impressions in relief of the 

 footprint, which, though rudely reproducing the form of the foot 

 which made the impression, reproduces the impression in reverse. 

 Since the original fossil is the impression (of which there may be 

 niany made by one individual) and not the animal's foot, the relief 

 impression of it must be considered a mold and not a cast. 



Trails. These are made by animals crawling over the mud and 

 dragging their bodies along. Jelly-fish floating in shallow water 

 may have their tentacles dragging along over the bottom, thus 

 leaving distinct impressions. Plants are not infrequently dragged 

 along over the shallow sea-bottom, with the result that a certain 

 type of trail is made on the mud which may be indistinguishable 

 from similar trails made by floating animals. Even attached plants, 

 like the beach grass on the sand-dunes, may have a very charac- 

 teristic semi-circular trail when swung about by the wind. Sea- 

 weeds partly buried on an uncovered mud flat may be moved by 

 the wind and so produce similar markings. These may, in some 

 instances, be preserved, as appears to have been the case in the 

 structures described as Spirophyton from the Palaeozoic rocks of 

 North America and elsewhere. 



Biirrozi's. While tracks and trails are made by animals in 

 transit, burrows are the temporary or permanent abodes of animals. 

 At the end of many trails of molluscs or Crustacea a mound is 

 found which marks the place where the creature has temporarily 

 buried itself in the sand. This type of burrow is not generally 

 well preserved, tiiough under favorable conditions it may be found. 

 At the end of a peculiar trail on the Potsdam sandstone of New 

 York, known as Climactichnites, Woodworth (29) has discovered 

 an oval impression which he considers to have been made by 

 the animal in resting. This may possibly represent the collapsed 

 burrow. 



The remarkable structures known as Da?monelix which occur 

 in the Miocenic deposits of the Nebraska region and which were 

 first described as sponges and have often been considered as plants, 

 are probaljly the burrows of some species of !)urrowing mammal. 

 The strata in which they occur are of the continental type of dc- 



