IOWA ACADEMY OF SCIENCES. 
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in a burrow. In addition, cross-setting and polishing or 
etching the cut surface fails to bring out any circular 
boundary lines of burrows, such as we should expect the 
limestone to preserve. 
Still the animal could hardly be expected to erect itself 
on its pygidium, and at first blush it seems ridiculous to 
suppose that it had the power to press itself backward into 
the soft mud. But that seems the only tenable theory. It 
is supported by the character of the pygidium of Nileus 
vigilans , broadly wedged-shaped, stout, and entire of 
margin. It is likewise supported by the fact that some 
modern crustaceans have a similar habit. 
The facts observed would indicate that this group of 
trilobites were voluntarily buried posteriorly, and that 
anteriorly they kept their eyes above the surface of the 
sediment until, as it rapidly accumulated, they met their 
death, and were buried by the next layer of rock-forming 
material. 
