186 PALAEONTOLOGY. 



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. 82, 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' } e", in Protichnites 8-notatus. 



Yet, with respect to the hypothesis 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 

 animal. So that the most probable conception is, that the 

 creatures which have left these tracks and impressions on the 

 most ancient of known sea-shores belonged to a crustaceous 

 genus, either with three pairs of limbs employed in locomotion, 

 and severally divided to accord with the number of prints in 

 each of the three groups, or bifurcated merely, the supplemen- 

 tary and usually smaller impressions being made by a small 

 and simple fourth, or fourth and fifth pair of extremities. 



The great entomostracous king-crab (Limulus) which has 

 the small anterior pair of limbs near the middle line, and the 

 next four lateral pairs of limbs bifurcate at the free extremity, 

 the last pair of lateral limbs with four lamelliform appendages, 

 and a long and slender hard tail, comes nearest to the above 

 idea of the kind of animal which has left the impressions on 

 the Potsdam sandstone. 



The shape of the pits, so clearly shewn in the ice-rubbed 

 slabs, impressed by Protichnites 8-notatus, accords best with 



