224 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mar. 24, 



in PI. IX., the outer impression, c\ is single, but in the preceding set 

 it is divided : whilst the impressions «, a\ are confluent in that set, 

 and are separate in 1 i. The same variety occurs in the outer pair, 

 e\ c", in Protichnites S-notatus. 



Yet, with respect to the hypothesis that each impression was made 

 by its own independent limb, I confess to much difficulty in con- 

 ceiving how seven or eight pairs of jointed limbs could be aggregated 

 in so short a space of the sides of one animal. So that I incline to 

 adopt as the most probable hypothesis, that the creatures which have 

 left these tracks and impressions on the most ancient of known sea- 

 shores belonged to an articulate and probably 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 supplementary and usually smaller 

 impressions being made by a small and simple fourth, or fourth and 

 fifth pair of extremities. 



The LimuluSf 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 the nearest to my 

 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 Protichnites 8-notatus, accords best with the hard, sub- 

 obtuse, and subangular terminations of a crustaceous 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 Macrura and Xipho- 

 suray and not sideways, like the Brachyurous Crustaceans. 



The appearances in the slab impressed by the Protichnites multino- 

 tatus favour the view of the mechan track having been formed by a 

 caudal appendage, rather than by a prominent part of the under sur- 

 face of the trunk. 



What further conjectures the contemplation and comparison of the 

 several series of foot-prints from the Potsdam sandstone have originated 

 in my mind, I do not deem it very helpful to their full understanding 

 at present to record. 



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 the 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 

 Plesiosaur and Ichthyosaur are instances in the reptilian class, and 

 the Pterichthysy Coccosteus^ and Cephalaspis in that of fishes. If 

 the Vertebrate type has undergone such inconceivable modifications 



* [This paragraph was added whilst the paper was being prepared for the press. 

 May 13, 1852.— R.O.] 



