158 JSFotes 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, Galymene, 

 JPhacops, and Spherocoryphe. The trilobites, without excep- 

 tion, 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, ami 

 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,"* 

 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 up- 

 wards, 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 spe- 

 cies. 



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 tlvan the nat- 



* Page 52, conclusions 1, 2, i. 



