1852.] OWEN POTSDAM SANDSTONE FOOT-TRACKS. 215 



But what strikes the ichnologist, heretofore conversant chiefly with 

 the foot- prints of bipeds or quadrupeds, is the occurrence in the pre- 

 sent series of the third impression, a, which complicates the most 

 approximated pair, being placed in front and a little to the inner side 

 of the innermost impression, a\ of that pair. The superadded im- 

 pression, «, is about the same size as the innermost in each pair, the 

 average diameter of that impression being 5 lines. 



Taking this view of the impressions, it appears that, whilst the in- 

 nermost in each pair, a', 5, c, are of equal size, the outermost, a", h\ c', 

 1 i, progressively increase in size, from the most approximated to 

 the most divergent of the three pairs ; that of the first, «", being 

 narrow in proportion to its length, that of the second, h\ as broad as 

 long, and the outermost, c', c", of the third pair being oblong, but larger 

 than that in the first pair. In some places where the most approxi- 

 mated pair of impressions, «', a", are deeply marked, they are compli- 

 cated by a fourth shallow and very small pit, «'", 2 X, midway between 

 the third, «, and the outermost, a}\ of the pair of impressions. 



The deepest parts of the middle track usually occur between the 

 second, h, b\ of each of the three groups of foot-prints. 



The first pair of impressions, «', o!', are included within a space of 

 1 inch 3 lines in diameter ; the third pair within a space of 1 inch 

 9 lines in diameter. The longitudinal extent of the three groups of 

 impressions, measured along the outermost, averages 3 inches 6 lines, 

 and along the innermost 3 inches 3 lines : the extreme extent of the 

 three sets of impressions averages 4 inches. The transverse interval 

 between the innermost impressions, ft, a, of the first pair is 3 inches, 

 and between those of the third pair, c, c, 2 inches, measured from 

 their innermost borders. The distance between the two outermost 

 impressions of the first pair is 5 inches, and it is the same between 

 the corresponding impressions of the third pair, measured from their 

 outer borders ; so that a line drawn along the outer margin of the 

 impressions of one side would be parallel to the line drawn along the 

 same parts on the opposite side, the difference in the distance from 

 the midspace being presented by the innermost impressions. 



The average breadth of the median groove is 5 lines, its depth at 

 the deepest parts between 1 and 2 lines ; the regular alternation of 

 the deep and shallow parts of the median impression indicates the 

 part that made it to have been alternately raised and depressed, an 

 alternation which might affect a tail as well as the trunk, but is more 

 likely to have affected the latter in an undulating mode of pro- 

 gression. 



There are no clear or unequivocal marks of toes or nails on any of 

 the impressions which form the lateral pairs or triplets. Their mar- 

 gins are not sharply defined, but are rounded off and sink gradually 

 to the deepest part, which is a little behind the middle of the de- 

 pression. There is a slight variation in the form and depth of the 

 answerable impressions, but not such as to prevent their correspond- 

 ence being readily appreciable through the whole of the extent here 

 described ; that is to say, the innermost of each of the three pairs 

 here described as first, A, second, B, and third, C, may be identified 

 with the corresponding innermost impression on the opposite side and 



