GEOLOGY. 305 



inches wide, and altogether sixty-one feet long ; the fourth, five tracks 

 from three-quarters of an inch to five and a-half inches wide, and 

 giving an aggregate length of eighteen feet ; and another area in the 

 next field has ten tracks of four to six and a-h?.lf inches wide, with a 

 total length of fifty-six feet. 



Prof. Owen, of London, in a recent communication to the Geolog- 

 ical Society, stated that he had been enabled to distinguish five well- 

 marked varieties of imprints, to which, for the sake of convenience, 

 he has given the following appellations : Proticlmites septemnotatits, 

 P. octonotatus, P. multinotatus, P. Uneatus, and P. alternans. After an 

 elaborate detailed description of the several tracks, (which have cer- 

 tain characters in common, such as a more or less regularly marked 

 longitudinal furrow, accompanied on either side by numerous closely 

 set imprints,) the Professor proceeded to observe, that, from their 

 peculiar arrangements, neither to a quadrupedal creature nor a fish- 

 like animal could these imprints be assigned ; and yet, said the Pro- 

 fessor, with respect to the hypothesis, that each imprint was made by 

 its independent limb, I confess to much difficulty in conceiving how 

 seven or eight pairs of jointed limbs cculd be aggregated in so short a 

 space of the sides of the 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, each limb having its extremity either 

 divided into three or more processes, or bifurcate merely, some of the 

 imprints, described as " supplementary," and usually of smaller size, 

 being made by a small and simple fourth, or fourth and fifth, pair of 

 limbs'. The shape of the pits in one of the slabs accords best with the 

 hard, sub-obtuse, and sub-angular terminations of a crustaceous ambu- 

 latory limb, such as may be seen in the blunted legs of a large Palinu- 

 rus or Birgus ; and it is evident that the animal of the Potsdam sand- 

 stone moved directly forwards, after the manner of the Macrura, and 

 not sideways, like the Brachyurous crustaceans. One specimen' favors 

 the supposition of the median groove having been formed by a caudal 

 appendage, rather than by a prominent part of the under surface of 

 the trunk. With reference to the conjectures that might be formed 

 respecting the creatures that have left these tracks, the Professor 

 observed, that the imagination is baffled in the attempt to realize the 

 extent of time passed since the period when these creatures were in 

 being that moved upon the sandy shores of the Silurian sea, and we 

 know that, with the exception of the most microscopic forms, all the 

 actual species of living beings disappear at a period geologically very 

 recent in comparison with the Silurian epoch. The forms of animals 

 present modifications more and more strange and diverse from actual 

 exemplars as we descend into the depths of time past. Of this the 

 Plesiosaur and the Ichthyosaur are instances in the reptilian class, and 

 the Pterichtnys, Coccosteus, and Cephalaspis in the class of fishes. If 

 then the vertebrate type has undergone such inconceivable modifica- 

 tions during the secondary and Devonian periods, what may not have 



