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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 14, 



by the spinous feet or abdomen of some crustaceous animal. As there 

 is every reason to believe that Trilobites had soft feet, they could not 

 have made such indentations with these instruments, and as the 

 only species known in these beds — the Olenus micrurus — had an 

 obtuse rounded tail, it could not have made the marks with that. 

 We must look then to the possibility of their having been made by 



the Hymenocaris, a shrimp-like animal, whose abdomen does possess 

 the requisite sharp prongs at its extremity, having in particular one 

 large spine Avhich seems to be exactly the instrument fitted for the 

 purpose. It is well known, that, when in rapid or violent movement, 

 the Crustacea can strike vigorously with the abdomen, and so gain the 



