How is the Cambrian divided ? 483 
foundland Mr. Walcott has found Olenus, and in the St. John 
area (Acadia) Leptoplastus occurs. In the latter area also 
the fourth Cambrian fauna has been found, being indicated 
by the presence of Ctenopyge flagillifer, C. spectabilis and 
Orthis lenticularis. 
A fuller presentation of Upper Cambrian forms is that 
which is found in the Mississippi valley in the states of Wis- 
consin and Iowa, where there is a succession of 600 feet of 
sandstones whose fauna has been described and figured by 
Dr. D. D. Owen and Prof. Jas. Hall. The latter divides this 
series into three parts, the lowest of which contains forms 
similar to those at the base of the Olenus zone in Europe. 
In the middle division, which is most prolific of the re- ~ 
mains of trilobites, are species which may be compared to 
those of the genera Olenus, Parabolina, Leptoplastus ? Eul- 
oma and Conocephalites. Dr. Dames compares others to 
Anomocare. It is only in the highest Potsdam division and 
in the beds above it, according to Prof. Hall, that the typical 
Dicellocephali appear, and these in Kurope are found in the 
Tremadoe or fifth Cambrian fauna. Triarthrella occurring 
in Wisconsin with these Dicellocephali is compared by 
Brégger to Cyclognathus, a genus of the fourth fauna and of 
the base of the Ordovician. The whole series of 600 feet 
in Wisconsin seems to belong to the Upper Cambrian. But 
the phase of the fourth Cambrian fauna represented in 
Europe and Acadia by Ctenopyge and its allies is absent, 
probably from the want of favorable habitat. 
Comparative bulk of measures holding the faunas. 
The relative age and position of the Paradoxides beds in 
the Cambrian system may be shown by the bulk of the 
measures in the different parts of the system, With our 
present knowledge, this can be only imperfectly done, but 
the following is a comparison of the mass of deposits in 
three different countries. When the system has been more 
carefully studied in different parts of the world a more exact 
proportion in the sedimentation will be had, 
