Notes on Ceraurus pleurexanthemus. 157 



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 show- 

 ing 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 posi- 

 tion ; 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 lycoperdoii) 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 associ- 

 ated with the Ceraurus, — every individual upon its back. 

 Cabjmene senaria, when not coiled (numerous), Proetus par- 

 viuscidus, Asaphus gigas (oue 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 



