ICHNOLOGY 161 



instruments, capable of being moved so far in advance as to 

 clear the previous impressions and make a series of new ones 

 at the same distance from them as the sets of impressions in 

 the series are from each other. What then was the nature of 

 these instruments 1 To this four replies may be given, or 

 hypotheses suggested : — They were made either, first, as in 

 the case of quadrupedal impressions, each by his own limb, 

 which would give seven and eight pairs of limbs to the two 

 species respectively ; or, secondly, certain pairs of the limbs 

 were bifurcate, as in some insects and crustaceans, another pair 

 or pairs being trifurcate at their extremities ; and each group of 

 impressions was made by a single so subdivided limb, in which 

 case we have evidence of a remarkably broad and short, and, 

 as regards ambulatory legs, hexapod creature ; or, thirdly, three 

 pairs of limbs were bifurcate, and the supplementary pits were 

 made by small superadded limbs, as in some crustaceans ; or, 

 fourthly, a single broad fin-like member, divided at its impress- 

 ing border into seven or into eight obtuse points, so arranged 

 as to leave the definite pattern described, must have made the 

 series of three groups by successive applications to the sand. 



The latter hypothesis appears to be the least probable, — 

 first, as being most remote from any known analogy ; and, 

 secondly, because there are occasional varieties in the groups 

 of footprints which would hardly accord with impressions left 

 by one definitely subdivided instrument or member. Thus in 

 the group of impressions marked 1 L in fig. 64, the outer 

 impression, c, is single, but in the preceding set it is divided ; 

 whilst the impressions, a, a, are confluent in that set, and are 

 separate in 1 L. The same variety occurs in the outer pair, 

 c, c", in Protichnites 8-notatus. 



Yet, with respect to the h) r pothesis that each impression 

 was made by its own independent limb, there is much diffi- 

 culty in conceiving how seven or eight pairs of jointed limbs 

 could be aggregated in so short a space of the sides of one 



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