ICHNOLOGY 1 63 



The deviations from the living exemplars of animal types 

 usually become greater as we descend into the depths of time 

 past ; of this the Archegosaur and Ichthyosaur are instances 

 in the reptilian class, and the Pterichthys and Coccosteus in 

 that of fishes. If the vertebrate type has undergone such 

 inconceivable modifications during the Secondary and Devo- 

 nian periods, what may not have been the modifications of the 

 articulate type 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 footprints indicate as to afford an 

 exact illustration of the shape and number of the instruments, 

 and of the mode of locomotion, of the Silurian Protichnites. 



Since the foregoing interpretation of the Silurian Ichnites 

 of North America was published, similar impressions have 

 been observed in rocks of the like high antiquity in Scotland, 

 as at Binks, Eskdale, which have received the name of Protich- 

 nites Scoticus* 



Amphibichnites. 



Genus Cheirotherium. — Fig. 68 gives a reduced view of a 

 portion of new red sandstone, with three pairs of footprints in 

 relief : the first and third of the left, the second of the right, 

 side. Consecutive impressions of such prints have been 

 traced for many steps in succession in quarries of that forma- 

 tion in Warwickshire and Cheshire, more especially at a 

 quarry of a whitish quartzose sandstone at Storton Hill, a 

 few miles from Liverpool. The footmarks are partly concave 

 and partly in relief; the former are seen upon the upper 

 surface of the sandstone slabs, but those in relief are only upon 

 the lower surfaces, being in fact natural casts, formed on the 

 subjacent footprints as in moulds. The impressions of the 



* Harkness and Salter " On the Lowest Rocks of Eskdale," Quarterly 

 Journal of the Geological Society, vol. xii., pp. 238, 243, fig. 2. 



