PROTICHNITES.
Protizhnites septem-notatue The subject so named consists of a series of well-defined impressions, continued in regular succession along an extent of 4 feet ; and traceable with an inferior degree of definition, along a further extent of upwards of 2 feet.
In the extent of 4 feet there are thirty successive groups of footprints on each side of a median furrow, which is alter nately deep and shallow along pretty regular spaces of about inches in extent. The number of prints is not the same in each group ; where they are best marked, as in fig. 64, 1 L, we see 3 prints in one group, a, a, a", 2 prints in the next, b, b', and 2 in the third, c, c, which is followed by a repetition of the group of 3 prints, a, a, a", making the numbers in the three successive groups 3, 2, 2 ; the three groups of impres sions being recognizably repeated in succession along the whole series of tracks on both sides of the median groove.
The principal footprints are disposed in pairs, placed with different degrees of obliquity, in each of the three groups towards the median track ; the innermost print in the second, B, and third, C, pairs, which are best marked, being usually rather more than half the size of the outer print, b' and c.
The two footprints of the same pair are a little further apart from each other, in the three succeeding pairs, as at a, a", b, b', c, c, especially in the second and third groups of each set ; the two forming the pair a', a", again approximating in the next series, and the pairs b, if and c, c, diverging in the same direction and degree ; and this alternate approximation • Quarterly Journal of the Geological Society, vol. viii., p. 214, 1852.
and divergence is repeated throughout the entire series of the present tracks.
But what strikes the ichnologist, heretofore conversant chiefly with the footprints of bipeds or quadrupeds, is the occurrence in the present series of the third impression a, Protichnita 7 -notatus (Cambrian).
which complicates the most approximated pair A, being placed in front and a little to the inner side of the hindmost impres sion, a", of that pair. The superadded impression a, is about the same size as the innermost in each pair, the average diameter of that impression being 5 lines.
Taking this view of the impressions, it appears that whilst the innermost in each pair, a, 1), c, are of equal size, the outer most, a", b ' , c , 1 L, progressively increase in size, from the most approximated to the most divergent of the three pairs ; that of the first, a", being narrow in proportion to its length, that of the second b', as broad as long, and the outermost, c, c", of the third pair being oblong, but larger than that in the first pair. In some places where the most approximated pair of impressions, a, a", are deeply marked, they are complicated by a fourth shallow and very small pit, a"', 2 L, midway between the third, a, and the outermost, a", of the pair of impressions.
There are no clear or unequivocal marks of toes or nails on any of the impressions which form the lateral pairs or triplets. Their margins are not sharply defined, but are rounded off, and sink gradually to the deepest part, which is a little behind the middle of the depression. There is a slight variation in the form and depth of the answerable impressions, but not such as to prevent their correspondence being readily appreciable through the whole of the extent here described ; that is to say, the innermost of each of the three pairs here described as first, A, second, B, and third, C, may be identified with the corres ponding innermost impression on the opposite side, and with the same impression of the same pair in the three preceding and the three succeeding pairs.
The impressions selected for fig. 64 clearly demonstrate that the animal, progressing in an undulating course, made at each action of its locomotive members, answering to the single step of the biped and the double step of the quadruped, not fewer than, in Protichnites 7-notatus, fourteen impressions, seven on the right and seven on the left ; and in Protichnites 8-notatus, sixteen impressions, eight on the right and eight on the left ; these seven and eight impressions respectively being arranged in three groups—viz., in Protichnites7-rwtatus, three, two, and two ; in Protichnites 8-notatus, three, two, and three —the groups being re-impressed, in successive series, so similarly and so regularly as to admit of no doubt that they were made by repeated applications of the same impressing instruments, capable of being moved so far in advance as to clear the previous impressions and make a series of new ones at the same distance from them as the sets of impressions in the series are from each other. What then was the nature of these instruments ? To this four replies may be given, or hypotheses suggested :—They were made either, first, as in the case of quadrupedal impressions, each by his own limb, which would give seven and eight pairs of limbs to the two species respectively ; or, secondly, certain pairs of the limbs were bifurcate, as in some insects and crustaceans, another pair or pairs being trifurcate at their extremities ; and each group of impressions was made by a single so subdivided limb, in which case we have evidence of a remarkably broad and short, and, as regards ambulatory legs, hexapod creature ; or, thirdly, three pairs of limbs were bifurcate, and the supplementary pits were made by small superadded limbs, as in some crustaceans ; or, fourthly, a single broad fin-like member, divided at its impress ing border into seven or into eight obtuse points, so arranged as to leave the definite pattern described, must have made the series of three groups by successive applications to the sand.