544 PROCEEDINGS OF THE ACADEMY OF [NOV., 



could all be permanently preserved in places where the rivers are 

 bringing considerable sediment into the water; where the fiats are 

 less frequently covered by the water, as, for instance, in the case of 

 the flood plains in the estuary of a river, they would have more 

 chance to be permanently preserved. From observations made 

 elsewhere, where a river that was subject to freshets after heavy 

 rains covered such flood grounds, the sediment after one rise would 

 often amount to one-quarter of an inch of silt, which would dry to 

 about one-third of this thickness. Such a layer would be quite 

 enough to cover and preserve all but the deeper tracks; and even 

 these might be filled in some cases. But the permanence and 

 sharply defined character of the impressions were the most signifi- 

 cant facts observed at this locality. 



The Trails and other Impressions on the Clay Flats. — In his 

 "Ichnology of New England," 1858, and the "Supplement to the 

 Ichnology of New England," 1865, Edward Hitchcock has de- 

 scribed, besides undoubted reptilian and batrachian tracks, many 

 other markings which he observed on the shale and sandstone layers of 

 the Connecticut Triassic. Some of them he ascribes to fishes, Crus- 

 taceans, Annelids, and insects, as well as some which he calls "fu- 

 coids, " or simply "plants," and a few that he did not assign to any 

 group. The illustrations of these irregular markings ascribed to 

 fishes, such as his genus Ptilichnus, as well as some that he calls 

 "of doubtful character," such as his Grammiehnus and /Enigmichnus, 

 and some of the "crustacean" and "insect" tracks, are very much 

 like forms that I observed on some of the smoother parts of these 

 flats. Continuous trails, such as he calls annelid tracks, were also 

 common, and in all cases the manner of production of these trails 

 was observed. They were not here produced by living animals 

 in any case. For instance, when the incoming tide carried along 

 with it a bunch of Ulva enieromorpha attached to a small gravel 

 stone, it might make a continuous line in the clay, as shown in 

 Plate XLII. This line would take any direction, depending upon the 

 eddies in the water; it would even double upon itself and the tracks 

 would cross. When the tide was receding the movements were 

 often more linear and the trails left were much straighter. Those 

 photographed were obtained after the tide had receded and the 

 bunches were left stranded. The Ulva enieromorpha grows in tufts 

 of some six to eight inches high, attached to stones and pebbles 

 which serve to anchor it. As the plant increases in size, the flotation 

 of the bushy fronds becomes, in moving water, sufficiently strong to 



