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THE POPULAR SCIENCE MONTHLY. 



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 



Fig. 6. 



* Hamipes. 



a single mark. Or the mud may have varied in its capacity for retain- 

 ing the impressions, so that one or more rows may be wanting. Such 

 cases are common among the Ichnozoa, so that track-ways very dis- 

 similar 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 fre- 

 quenting the sea-shore at low tide. 



Considering the lightness of these insects, or whatever animals 



Fig. 7. 



The PEOTOTicHNraEs. 



they may have been, it is remarkable that any part of these impres- 

 sions should have been preserved. Only one good locality of them is 

 known, upon a very fine-grained clayey rock, whose liquidity was suf- 

 ficient 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. 



