On seeing this great number of trilobites, that on account of their organization must be considered as blind, the first suggestion that strikes the mind, is that they must have lived in abyssal depths of the Cambrian sea, where the most intensive darkness prevailed. Nor does the nature of the strata contradict such an assumption as evidently this fine sediment must have been deposited far beyond the reach of the influence of the wave motion, a depth amounting to more than a thousand metres as now calculated, for else it could not have preserved unbroken such delicate parts of organisms as that free cheek of Ctenopyge pecten with its extraordinary long and delicate horn, figured on plate III fig. 27 and many others. But it seems incredible that such a state of things should have prevailed during the deposition of all the Cambrian strata, although they in Sweden amount to only 160 feet in thickness, according to the evaluation of S. A. TULLBERG, not considering what has been lost through denudation. The length and duration in time can in this instance not be measured by the thickness of the beds, but by the great changes in the faunas which there have succeeded one another. The physical conditions, to judge by the composition of the rocks, seem in the main to have lasted during immeasurable periods and still the fauna has changed in no little degree. During that enormous length of time, embracing in Sweden eight well separated periods, there must, however, have been minor changes in the conditions of depth and consequently in the nature of the depositions. To a certain extent the physical agents must have influenced the organization of the animals, but not essentially. There are sure evidences of another factor being the chief agent and that is the evolution. We have from the lowest Olenellus beds to the lowest Lower Silurian strata followed the clear traces of the changes from the eyeless Olenellidæ past the Paradoxidæ with the facial suture to the great tribe of the Olenidæ in which the facial ridge with its lobe is so prominent, and to the oculate genera at the top of the formation. As stated above there seems to be evidence enough for accepting two different lines of evolution in these the oldest trilobites. For the oldest, Olenellus is the type and for the second Sao may be taken as the representative. These two evolutional lines could be represented with their phases in chronological succession as follows: A. =The Olenellus line.= 1. The trilobite consisting only of the head with rhachis and pleuræ, no sutures, no ridge, no eyes. Adult forms precambrian. Corresponding larval forms (Olenellus asaphoides) in the Lowest Cambrian. Still older and simpler forms may be presupposed as preceding these.