Ceraurus pleurexanthemus. 159 



ural position of the living animal, which must, therefore, have 

 been with the back downward. 



The Asaphus is more frequently broken ; but the finest and 

 most perfectly preserved specimens, Avith but few exceptions, 

 are found on their backs. 



That portion of the fourth conclusion in reference to trilo- 

 bites living gregariously in vast numbers, is true of Ceraurus 

 pleurexanthemus, Asaphus gigas, and A. megistos, as found 

 in the stratum mentioned. 



Note. To October ltJth, 1875, 1160 specimens of Ceraurus 

 pleurexanthemus have been noted on the under surface of the 

 thin layer ("Ceraurus layer"). Of these 1110 lay on their 

 backs ; while but fifty presented the dorsal surface up. 

 Forty-tive of these fifty were very small, the remaining 

 five of medium size. 



XVIII. — Description of the Interior Surface of the Dorsal 

 Shell of Ceraurus pleurexanthemus, Green. 



By c. d. walcott. 



Read June 7, 1875. 



This interesting species, which has been referred to in the 

 preceding pages, has already been described by earlier writ- 

 ers,* as regards the general features of its structure and the 

 outer surface of its shell. In this article, therefore, I shall 

 omit all detailed reference to any of these points, and con- 

 fine the description, as closely as may be, to the inner, or 

 ventral, surface of the dorsal shell. This description is de- 



* Green, Monograph of Tnlobites, 1SJ2, page 81. fig. X. Hall, Palaeontology N. Y., 

 vol. I, page 242. 



