MARKINGS, FOOTPRINTS, AND FUCOIDS 321 



Under the carapace are several pairs of jointed limbs differing 

 in size and form. The two anterior are small and peculiarly 

 formed claws, used apparently in manipulating the food. The 

 four next are larger in size, and are walking feet, each furnished 

 with two sharp points which form a pincer for holding. The 

 last pair is much larger and stronger than any of the others, and 

 armed not only with a pair of pincers, but with four blunt nail- 

 like points. Under the abdomen are flat swimming feet, as they 

 have been called, each composed of a broad plate notched and 

 divided in the middle. When at rest these lie flat on each 

 other, but they can be flapped back and forth at the will of the 

 animal. 



Let us now see what use the creature can make of these 

 numerous and varied pedal appendages, and for distinctness' 

 sake we shall call the anterior set thoracic and the posterior 

 abdominal. When placed in shallow water on fine sand it 

 walked slowly forward, and its tracks then consisted of a 

 number of punctures on the sand in two lines. If, however, the 

 water was very shallow or the sand very soft or inclined upward 

 the two edges of the carapace touched the bottom, making a 

 slight furrow at each side ; and if the tail was trailed on the 

 bottom, this made a third or central furrow. When climbing a 

 slope, or when placed at the edge of the water, it adopted 

 another mode of locomotion, pushing with great force with its 

 two posterior limbs, and thus moving forward by jerks. It 

 then made four deep marks with the toes of each hind limb, 

 and more or less interrupted marks with the edges of the cara- 

 pace and the tail. In these circumstances the marks were al- 

 most exactly like those of some forms of the Protichnites of the 

 Potsdam sandstone. When in sufficiently deep water and de- 

 sirous to escape, it flapped its abdominal feet, and then swam 

 or glided close to the bottom. In this case, when moving near 

 the soft bottom, it produced a series of transverse ridges and 

 furrows like small ripple marks, with a slight ridge in the middle, 



