Page:Ann. Lyc. Nat. Hist. 11 155-162.djvu/4

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Notes on Ceraurus pleurexanthemus.

the succeeding layer, or imbedded in the clayey shale, were bryozoans, cystids, crinoids, brachiopods, and trilobites of the following genera:—Ceraurus, Acidaspis, Calymene, Phaeops, and Spherocoryphe. The trilobites, without exception, were back-down. In the succeeding layer, which is six inches thick, many of Asaphus gigas and A. megistos have been found, scattered through the lower three inches of its thickness. Of seventy-five noted, thirty were back-down, twenty-nine presented the dorsal surface up, sixteen were in various positions, coiled, perpendicular to the layer, and edgeways. The succeeding five feet of the stratum is of the same nature as that below. Fossils are rare, especially trilobites. Above this, the coarse earthy limestone extends to the thick crystalline strata.

Prof. Burmeister, in his "Organization of Trilobites,"[1] gives the following among other conclusions, as the result of comparison of the trilobites with the recent crustacea.

1. That these animals moved only by swimming; that they remained close beneath the surface of the water; and that they certainly did not creep about at the bottom.

2. That they swam in an inverted position, the belly upwards, the back downwards, and that they made use of their power of rolling themselves into a ball, as a defence against attacks from above.

4. That they most probably did not inhabit the open seas, but the vicinities of coasts, in shallow water; and that they here lived gregariously in vast numbers, chiefly of one species.

If the first and second conclusions are correct, we should look in a quiet, undisturbed deposit for evidence as to the position of the trilobites while living, by their position when buried in the sediment after death. As before stated, the conditions are such in the species mentioned, as to preclude the idea of their arrangement by other causes than the nat-

  1. Page 52, conclusions 1, 2, 4.