216 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mar. 24, 



with the same impression of the same pair in the three preceding and 

 the three succeeding pairs. 



This power of determining the homologies, so to speak, of the se- 

 veral impressions is a strong evidence of their having been made by 

 the successive application of the same instruments ; whilst the equal 

 distances at which they recur proves them to have been made in re- 

 gular succession, as in the ordinary progression of an animal walking 

 by means of limbs. 



The question next to be resolved is, — how those instruments were 

 disposed in the body of the creature that made and left the impressions? 



It cannot be supposed that two limbs, answering to the fore and 

 hind legs of a quadruped, could have made impressions so different in 

 form and in their degrees of approximation as we see in each pair of 

 the series of three sets on one side. In a quadruped we are accustomed 

 to see the successive pairs of the same side resembling each other, 

 the difference in the two impressions of such pairs indicating the 

 difference between the fore and hind feet of the side to which they 

 belonged ; but in the present series of impressions each pair in the 

 successive series of three so differs from the other two pairs in the 

 form and size of the impressions, and their degree of divergence, as 

 to render it scarcely possible to suppose that they could have been 

 formed, either along the inner or the outer series of impressions, by 

 successive steps of the same limb ; and, were it contended that the 

 animal by some peculiarity of gait more and more approximated its 

 fore limbs in making three successive steps, and then divaricated them 

 to commence another series of three steps, on the supposition that 

 the inner impressions were formed by the same pair of fore limbs at 

 each series of three steps, the difficulty would still remain of account- 

 ing for the third superadded impression, «, on the hypothesis of their 

 being formed by a quadruped, with the additional difficulty of the 

 difference in shape and size of the outer impressions of the same 

 three pairs. 



The first or most approximated pair, a', «", in each set of three 

 pairs of impressions are the most equal in size. In the second pair, 

 h, V, which are nearly equally approximated, the outer impression is 

 manifestly larger and broader than the inner one. In the third and 

 most divergent pair, c, c', the outer one is still larger, in length as well 

 as in breadth, and is occasionally subdivided. Now, as the first pair in 

 each series of three. A, 1 i, plainly answers to the same pair in the 

 next series of three. A, 2 i, and the like in regard to the second pair 

 and the third pair, it follows that the same instruments must have 

 made the first pair in each of the three pairs, and so of the second and 

 of the third pairs ; or, in other words, each pair in a series of three 

 must have been made by different members, or divisions of mem- 

 bers ; and the same must be inferred in respect to the small impres- 

 sion, 05, which is superadded to the first pair in each triplet : whence 

 it may be concluded either that the animal had seven pairs of ambu- 

 latory limbs, or that it had three pairs, of which two were bifid and 

 the third trifid at the impressing extremity. 



The impressions which are so clearly marked in the extent above 

 described are continued less distinctly but uninterruptedly for more 



