1911.] NATURAL SCIENCES OF PHILADELPHIA. 545 



move the small pebbles, up to three-quarters of an inch in diameter, 

 for instance. Where these -are caught by the incoming tide they 

 are carried along in the water at a rate that depends upon the drag 

 of the anchor. Observations were made upon them in water varying 

 from six to ten inches deep, by wading out on the flats from the 

 shore, and in deeper water by observing the motion from a boat. 

 The fronds of the Ulva stood upright from the anchoring pebble 

 and waved back and forth with the passage of the wind waves. 

 But as the fronds are exceedingly nexuous, the anchor was not 

 lifted from the bottom, as a rule, by the passage of a wave; the 

 frond simply expanded and collapsed, or rose and fell, with the up 

 and down movement of the surface of the water; and as far as the 

 graving action of the pebble was concerned, it was continuous or 

 nearly so. The trails made were more or less deeply impressed 

 according as the muddy surface was more or less yielding, and also 

 according as the anchor was pressing with full force or with a dimin- 

 ished pressure, as the frond of the Ulva was extended or collapsed 

 by the movement of the waves. In some cases the trails were made 

 by the advancing plant being dragged towards shore by the incoming 

 tide, in other cases by the receding tide carrying it away from shore. 



The form of the trail that was left depended upon the shape of 

 the pebble. Thus some were simply single concave grooves, the 

 sides being raised a little above the general surface of the mud, and 

 these look like what are called molluscan or annelid trails in such 

 structures when found fossil. They are very much like Hitchcock's 

 Unisulcus. Others were double grooves, when the shape of the 

 anchoring pebble was more irregular or when it had a groove in its 

 lower side. These were like Hitchcock's Bisulcus. It should be 

 borne in mind that the Ulva is attached to one point on the anchoring 

 pebble, and hence that one side of the pebble is always uppermost, 

 so that the surface which makes the impressions is always the same 

 one. 



Hitchcock's genus Ptilichnus is represented by a track which 

 consists of a very irregular series of markings that recur at regular 

 intervals, but that could not be made by the feet or other appendages 

 of passing animals. He assigns them to markings made by fishes. 

 Of these tracks he describes several species, distinguished by the 

 varying forms of the tracks. Such are Ptilichnus anomalus, P. 

 pectinatus, P. hydrodromus, etc. Another such track is what Hitch- 

 cock calls Saltator, under the impression that the regularly recurring 

 impressions were made by some leaping animal. All of these, 



