The latter hypothesis appears to be the least probable,— fust, as being most remote from any known analogy ; and, secondly, because there are occasional varieties in the groups of footprints which would hardly accord with impressions left by one definitely subdivided instrument or member. Thus in the group of impressions marked 1 L in fig. 64, the outer impression, c, is single, but in the preceding set it is divided ; whilst the impressions, a, a', are confluent in that set, and are separate in 1 L The same variety occurs in the outer pair, e', e", in Protichnite8 8-notatus.
Yet, with respect to the hypothesis that each impression was made by its own independent limb, there is much diffi culty in conceiving how seven or eight pairs of jointed limbs could be aggregated in so short a space of the sides of one animal. So that the most probable conception is, that the creatures which have left these tracks and impressions on the most ancient of known sea-shores belonged to a crustaceous genus,—either with three pairs of limbs employed in locomotion, and severally divided to accord with the number of prints in each of the three groups,—or bifurcated merely, the supplemen tary and usually smaller impressions being made by a small and simple fourth, or fourth and fifth pair of extremities.
The great entomostracous king-crab (Limulus) 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 nearest to the above idea of the kind of animal which has left the impressions on the Potsdam sandstone.
The shape of the pits, so clearly shown in the ice-rubbed slabs, impressed by Protichnites 8-notatus, accords best with the hard, subobtuse, and subangular terminations of a crusta ceous ambulatory limb, such as may be seen in the blunted legs of a large Palinurus or Birgus ; and it is evident that the animal of the Potsdam sandstone moved directly forwards after the manner of the Macroura and Xiphosura, and not sideways, like the brachyurous Crustaceans.
The appearances in the slab impressed by the Protichnites multi-notatus favour the view of the median track having been formed by a caudal appendage, rather than by a prominent part of the under surface of the trunk.
The imagination is baffled in the attempt 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 Silurian sea : and we know that, with the exception of certain 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 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.*