36 EXTINCT MONSTERS 



seven 011 the left, each time its legs were set to work. There seems 

 to be no doubt of this, because the groups of tracks, as marked 

 out in our illustration, occur again and again in successive series 

 so similarly and so regularly as to admit of no doubt that they 

 were made by repeated applications of the legs, and these must 

 have been capable of being moved so far in advance as to keep 

 clear of the previous group of impressions. Sir Eichard Owen 

 concludes his account of the tracks by saying that the creature 

 which made them was probably a crustacean genus, and that it 

 may have had three pairs of limbs employed in locomotion, each 

 of which was split up into two or more parts so as to make in 

 walking either two or three tracks. The shape of the pits so 

 clearly seen on these slabs of the old Potsdam Sandstone (although 

 they have been rubbed and polished by the action of glacier-ice) 

 suggests that they were made by the hard and partly pointed, 

 partly blunt, terminations of the limb of a crustacean, such as a 

 crab or lobster. But this creature moved directly forwards, not 

 like a crab, but like a lobster or a king-crab. The furrow that 

 runs between the tracks was probably made by a tail. The 

 question then arises what sort of a crustacean was it that made 

 these tracks ? Great caution is required in dealing with a problem 

 of this kind, as will be seen from the following words of Sir E. 

 Owen: "In all probability no living form of animal bears such a 

 resemblance to that which the Potsdam footprints indicate as to 

 afford an exact illustration of the shape and number of the instru- 

 ments, and the mode of locomotion of the Protichnites." The 

 imagination is baffled in the attempt to realise the extent of time 

 past since the period when the creature was in existence which 

 moved upon the sandy shores of the ancient Cambrian period, to 

 which the " Potsdam Sandstone " belongs. 



In about the year 1830, much interest was excited by the 



