Mode of Occurrence. 95 



confidant that the Trilobites may take a firm place at the root of 

 the Crnstacean system, with the existing Apus as their nearest 

 ally.* 



There is yet much to be learned from the study of Triarthrus. 

 A great amount of material can be readily collected at the local- 

 ity near Rome, N. Y. It is also of interest to note that the lo- 

 cality at Trenton Falls, N. Y., from which the specimens of 

 Calymene and Ceraurus were obtained, is only seventeen miles 

 from the Rome locality ; that both occur within the Ordovician ; 

 and that the stratigraphic position of the bed at Rome is be- 

 tween six and seven hundred feet above that at Trenton Falls.f 



*Nature, Vul. 48, 1893, p. 582. 



tTlie appendages of Triarthrus are replaced by iron pyrites and are asu- 

 ally well preserved. The specimens of Calymene and Ceraurus from the 

 Trenton limestone of Trenton Falls, N. Y., were replaced by calcite and in 

 them there were preserved even more delicate parts than I have yet ob- 

 served in Triarthrus. Thin sections were made of the latter and photo- 

 graphs obtained by transmitted light, that were used in illustratmg the 

 paper in the Bulletin of the Museum of Comparative Zoology, Vol. 8, 1881. 



