38 Fossils of the Potsdam Sandstone. 



aggregated in so short a space of the sides of one animal ; so that I ineline 

 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, be- 

 long to an articulate and probably crustaceous genus, either with three pairs 

 of limbs employed in locomotion, and generally divided to accord with the 

 number of prints in each of the three groups, or bifurcated merely, the supple- 

 mentary and usually smaller impressions being made by a small and simple 

 fourth, or fomih and fifth pair of extremities." 



'•' The Limulus, (King crab.) 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 left the impressions on the Potsdam Sandstone." 



He states that the animal mored forward, not sideways like some of the 

 crabs, and that in his opinion the median groove was formed by a caudal 

 appendage rather than by a prominent portion of the under part of the body. 

 " What further conjectures," says the learned professor, " the contemplation 

 and comparison of the several series of foot-prints from the Potsdam Sandstone 

 have originated in m.y mind, I do not deem it very helpful to their full 

 understanding at present to record. The imagination is baffled in the at- 

 tempt 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 Silu- 

 rian 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 de^dations from the 

 living exemplars of animal types usually become gi-eater as we descend into 

 the depths of time past ; and of this the Plesiosaur and Ichthyosam- are 

 instances in the reptilian class, and the Pierichthys, Coccosteusy and 

 Cephalaspis in that of fishes. If the Vertebrate type has undergone such 

 inconceivable modifications during the Secondary and Devonian periods,. 

 Avhat may not have been the modifications of the Articulate t}-pe 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 

 resemblance to that which the Potsdam foot-prints indicate, as to afford an 

 exact illustration of the shape and number of the instruments and of the 

 mode of locomotion of the Silurian Proiichnites. These most precious 

 evidences of animal life, locomotive on laud, of the oldest known sedimen- 

 tary and unmetamorphosed deposits on this planet, have been, I am well 

 aware, far too inadequately described in the paper which I have the honour 

 to submit to the Society. They offer characters which require more time 

 for their due scrutiny and greater acumen and powers of interpretation than 

 liave hitherto been bestowed upon them. The symbols themselves are dis- 

 tinct enough. Old Nature speaks as plainly as she can do by them ; and if 

 we do not fully thereby read her meaning the fault is in our powers of 

 interpretation. In the present attempt I can, however, truly aver that I 

 bestowed upon it all the leisure at my command, and have applied my best 



