ICHNOLOGY. 185 



ponding innermost impression on the opposite side, and with 

 the same impression of the same pair in the three preceding 

 and the three succeeding pairs. 



The impressions selected for fig. 82 clearly demonstrate 

 that the animal, progressing in an undulating course, made at 

 each action of its locomotive members, answering to the single 

 step of the biped and the double step of the quadruped, not 

 fewer than, in Proticlmites 7-notatus, fourteen impressions, 

 seven on the right and seven on the left; and in Proticlmites 

 %-notatvs, sixteen impressions, eight on the right and eight on 

 the left ; these seven and eight impressions respectively being 

 arranged in three groups — viz., in Proticlmites 7-notatus, three, 

 two, and two ; in Proticlmites 8-notatus, three, two, and three 

 — the groups being re-impressed, in successive series, so 

 similarly and so regularly as to admit of no doubt that they 

 were made by repeated applications of the same impressing 

 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 ? To 

 this three replies may be given, or hypotheses suggested : — 

 They were made either, first, as in the case of quadru- 

 pedal impressions, each by his own limb, which would give 

 seven and eight pairs of limbs to the two species respec- 

 tively; or, secondly, certain pairs of the limbs were bifur- 

 cate, as in some insects and crustaceans, another pair or pans 

 being trifurcate at their extremities; and each group of im- 

 pressions 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 hi some crustaceans ; or, 

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



