162 PALEONTOLOGY 



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 shown in the ice-rubbed 

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

 the hard, subobtuse, and subangular terminations of a crusta- 

 ceous ambulatory limb, such as may be seen in the blunted 

 legs of a large Palinurus or Birgus ; and it is evident that the 

 animal of the Potsdam sandstone moved directly forwards after 

 the manner of the Macroura and Xiphosura, and not sideways, 

 like the brachyurous Crustaceans. 



The appearances in the slab impressed by the Protichnites 

 multi-notatus favour the view of the median track having been 

 formed by a caudal appendage, rather than by a prominent 

 part of the under surface of the trunk. 



The imagination is baffled in the attempt to realize the 

 extent of time past since the period when the creatures were 

 in being that moved upon the sandy shores of that most 

 ancient Silurian sea : and we know that, with the exception 

 of certain microscopic forms of life, all the actual species of 

 animals came into being at a period geologically very recent 

 in comparison with the Silurian epoch. 



