Page:Popular Science Monthly Volume 3.djvu/452

438 front pair of limbs, and the outer by the hindmost. Such groups would be preserved only under the most favorable circumstances. In many species the three pairs of legs might be of the same length, and two or more of the feet might tread upon the same spot, leaving but



a single mark. Or the mud may have varied in its capacity for retaining the impressions, so that one or more rows may be wanting. Such cases are common among the Ichnozoa, so that track-ways very dissimilar to the unpractised eye are referred to the same species.

No attempt has been made to refer several ichnitic genera to the several orders of insects. With the small information now existing respecting insectean locomotion, such reference would be premature. It is very obvious that the selection must be made from groups frequenting the sea-shore at low tide.

Considering the lightness of these insects, or whatever animals



they may have been, it is remarkable that any part of these impressions should have been preserved. Only one good locality of them is known, upon a very fine-grained clayey rock, whose liquidity was sufficient to allow the marks to be made, and yet so solid that the heat of the sun evaporated the bulk of the water between the ebb and flow of a single tide. This locality is at the Lily Pond, Turner's Falls, near Greenfield, Massachusetts.