Page:Ann. Lyc. Nat. Hist. 11 155-162.djvu/3

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157
Notes on Ceraurus pleurexanthemus.

subjected to the action of the water in drifting them into the position found. The thorax is easily dismembered and broken, and could not have withstood such transportation.

2. Very few fragments are found, and when consisting of the head or pygidium, they have the dorsal surface down.

3. Upon all uneven layers, and those showing the action of strong currents, and holding coarser fossils, the trilobites are distorted and broken up.

4. When found upon smooth layers above the Ceraurus layer, they are invariably back-down. Fine specimens showing the interior of the shell, are obtained from the upper surface of several layers.

5. The drifting of the shell into the position found, would not probably have taken place, as the shell is nearly flat. Asaphus might, from its boat-like shape, assume such a position; but a force sufficient to place a trilobite like Ceraurus upon its back, if the natural position when living was the reverse, would not have left the bryozoans and crinoids as they grew, without breaking the more delicate portions, which are often like fine hairs of stone, lying loosely in the imbedding clay, and breaking at the slightest touch.

6. The trilobites never have shells or corals drifted into them, or lying on them, when upon the upper surface of the layers. Occasionally a coral encrusts the upper surface, and frequently a coral (Stenopora lycoperdon) has taken the interior or ventral surface as a base for its growth, showing clearly that the shell had assumed the inverted position prior to the growth of the coral.

Forty specimens of Acidaspis Trentonensis were associated with the Ceraurus,—every individual upon its back. Calymene senaria, when not coiled (numerous), Proetus parviusculus, Asaphus gigas (one only), and Dalmanites, were uniformly back down.

Upon the upper surface of the Ceraurus layer, a layer of clayey shale was deposited, giving the same conditions as below the Ceraurus layer. Attached to the under surface of