ICHNOLOGY. 187 



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 Xvphomra, and not sideways, 

 like the brachyurous Crustaceans. 



The appearances in the slab impressed by the Proticlmites 

 multi-notahts 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. 



The deviations from the living exemplars of animal types 

 usually become greater as we descend into the depths of time 

 past ; of this the Archegosaur and Ichthyosaur are instances 

 in the reptilian class, and the Pterichthys and Coccosteus in 

 that of fishes. If the vertebrate type has undergone such 

 inconceivable modifications during the Secondary and Devo- 

 nian periods, what may not have been the modifications of the 

 articulate type during a period probably more remote from the 

 secondary period than this is from the present time ? In all 

 probability no living form of animal bears such a resem- 

 blance to that which the Potsdam footprints indicate as to 

 afford an exact illustration of the shape and number of the 

 instruments, and of the mode of locomotion, of the Silurian 

 Protichnites. 



Since the foregoing interpretation of the Silurian Ichnites 

 of North America was published, similar impressions have 

 been observed in rocks of the like high antiquity in Scotland, 



