36 EXTINCT. MONSTERS 
seven on 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 Richard 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 R. 
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 baftled 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 
