1911.] NATURAL SCIENCES OF PHILADELPHIA. 547 



though made by a punch, or they are elongated or even linear." 

 There are also " linear furrows nearly parallel to the other impressions, 

 but crossing at a small angle/' which were made, no doubt, by some 

 other object subsequently, perhaps during a different tide. 



He also describes another slab with Mniqmidinus on which were 

 "a trifid arrangement of somewhat triangular toes with two dents 

 behind and two or three on one side, and this arrangement is repeated 

 about once in an inch. The axis of the foot in this case is turned 

 aside from the line of direction as much as 30°, but I cannot decide 

 in which direction the animal was moving nor find a series of impres- 

 sions to the right or left corresponding to this one." 



This description (and the figure on Plate I, " Supplement") 

 represent very exactly the kind of impressions made by a rolling 

 piece of Fucus. Thus only the feet on one side of the hypothetical 

 animal make any impression, or else it must move sidewise, but the 

 impressions recur at perfectly regular intervals. That is exactly 

 the character of impressions made by an object rolling over the 

 bottom, and it is also the character of the impressions made by the 

 rollers in a print mill manufacturing cotton prints. That the " foot- 

 marks" were all right or all left might have suggested this analogy, 

 if the idea of a rolling object had occurred to Prof. Hitchcock. 



From the foregoing, it is evident that markings closely simulating 

 moll use an and annelid trails may be produced without animal 

 agency, and even such markings as may be mistaken for those made 

 by fishes, crustaceans or reptiles may be similarly produced. 



Explanation of Plates XLI and XLII. 



Plate XLT. — Fig. 1. — Rippled surfaces of the Leda clay on the flats northwest 

 of Dove Point, looking westward. 

 Fig. 2. — A nearer view of the erosion ripples on the clay flats, looking east 

 towards Dove Point. 



Plate XLII. — Fig. 1. — Trail made by a pebble attached to a tuft of Ulva entero- 

 morpha, and dragged over the bottom. The attached tuft of the Ulva may 

 be seen in a collapsed condition at the end of the trail. 

 Fig. 2. — A part of the clay flats, covered with pebbles with their attached 



Ulva tufts, that have left markings by their movement over the bottom. 

 Fig. 3. — A portion of fig. 2, enlarged, showing some of the markings in more 

 detail. 



