12 C. F. MATTHEW ON THE 



Jas. Hall divided the Potsdam saudstoue ofthe Mississippi Valley into three parts, a lower 

 which contained no Dicellocephali, a middle iu which they most abounded, and an upper 

 in which they are less common, but which contains the typical species discovered by 

 Owen. The middle part of the terraue containing the smaller Dicellocephali has a strictly 

 Cambrian fauna.' These either by genera or species indicate that part of the Cambrian 

 system which includes (the Upper Paradoxides ? and) the Olenus beds, i.e., measures 

 which lie below the Dict}'onema beds. 



The Dicellocephali found at Quebec lend no aid in determining whether the genus 

 was, in that part of America, existent in beds below those which carry Dictyonema, be- 

 cause, as has been already remarked, the examples have been found only in boulders, and 

 the parent bed remains unknown. 



But the Dicellocephali of the Eureka district in the Rocky Moiiutaius are associated 

 with a fauna which is strictly Cambrian, and its comparatively numerous species of 

 Agnosti would in Europe be considered to indicate a rather low horizon in the Cambrian. 

 Dicellocephalus was thus associated in America with strictly Cambrian genera. 



But in Europe it was otherwise. In Wales its remains are found in the Tremadoc 

 group above the horizon of Dictyonema. In these slates occur the genera Asaphus (var.), 

 Niobe, Cheirurus, and Cybele (which find their full development in the later Ordovician 

 age) and these genera are mingled with genera of Cambrian type. 



The Ceratopyge limestone and slate which in Sweden represent the Tremadoc grovip 

 and contain the Swedish Dicellocephali, have in addition to the genera named above, 

 others of Ordovician type, as Megalaspis, Remopleurides, Nileus, Ampyx and Amphion, 

 here also mingled with the fading representatives of the Cambrian fauna.' Dicello- 

 cephalus, therefore, which appeared in America in a purely Cambrian fauna, reached 

 Europe in association with trilobites which are familiar to us as leading genera of the 

 Ordovician fauna. In Europe it certainly succeeded Dictyonema, in America it probably 

 preceded it. 



In the interior of America no fauna similar to that of the Dictyonema beds of Europe 

 and Eastern Canada has been found. The species and many of the genera of the beds 

 which appear to be cotemporaneous, are such as do not accompany this graptolite where 

 it is known to have existed. It penetrated the valley of the St. Lawrence from the east as 

 far as Matane below Quebec, but though it thus nearly approaches the Canadian locality 

 of Dicellocephalus, its relation stratigraphically to the parent bed of Dicellocephalus in 

 Canada is unknown. It also has been found in the Island of Cape Breton and at St. John, 

 N.B., and thus belongs to the Atlantic area. In "Western Europe it had a wider distribu- 

 tion, having been found in Belgium and the northern part of France, as well as in Wales 

 and Scandinavia. 



When, as we shall see in the next section of this address, a graptolite fauna again 

 invaded the Valley of the St. Lawrence it was accompanied by a Dictyonema, differing in 

 type from the one aboA'e referred to. This modified form was D. delicalHhtm, Dawson. 



' Ptychoparia, Anomocare, Conocephalites, Pt\fliaspis, C'repicepbalus, Carioceplialus, Enlonia, Agnostiis, 

 Illi»nurus. 



' Such as Triarthrus, Ceratopyge, Euloma, Shumardia, Parabolinella, Cyclognathus, Agnostus, etc. 



