different one from that of the Olenellidæ and the Paradoxidæ. They have a rhachis, but no pleura proper, as the single facial ridge has a quite different signification and appears at a comparatively much later stage than the facial ridge of the Olenellidæ, which is present from the earliest stages known.[26] [Footnote 24: Fossiler fra Öxna og Klettna, Geol. För. Förhandl. 1875, p. 572, pl. 25, fig. I-X.] [Footnote 25: Foremost among these stands MATTHEW in »Illustrations of the Fauna of St. John» IV, where he, p. 143, pl. II figs. 1 f. etc., describes a few stages very like those given by BRÖGGER, so the glabella as an unsegmented, narrow ridge etc.] [Footnote 26: A deviating form of the ridges is shown by »Liostracus» tener HARTT, Acad. Geol. 2d Ed. p. 652 (see also MATTHEW Illustrations of the Fauna of St. John [1887] p. 132 p. 1 f. 3 a-3 c). Beside the usual facial ridge there is a second pair of ridges between the first and the glabella, arching in an opposite direction. See also WALCOTT, Bulletin U. S. Geol. Survey, N:o 10, 1884 pl. V, figs 6, 6 a, 6 b, new figures and copy of HARTT'S description.] This form of facial ridge, although prevalent in the middle and later Cambrian times, dates back so far that genera coeval with Holmia, viz. Ellipsocephalus and Arionellus show it along the facial suture. This early occurrence of the ridge coeval with the less developed Olenellidæ leads to the assumption of a different origin of these trilobites as a branch, which already far back in the oldest Cambrian or precambrian times had deviated from a common ancestor and I have tried to give a view in tables of these two coordinated lines of evolution further on, at pages 24 and 25. The most remarkable genera which, as far as I have found, belong to this fourth group are Ellipsocephalus, Arionellus, Liostracus, Olenus, Leptoplastus, Parabolina, Corynexochus, Parabolinella, Sao, Ptychoparia, Doropyge, Oryctocephalus etc. I have not been able with an absolute degree of certainty to recognize whether some of these genera now enumerated, have been oculate or blind like Sao, like the Paradoxidæ and similar. The precarious state of preservation prevents all definite conclusions in that respect. It seems, however, that the evidence gathered through the examination of numerous specimens rather points in a negative direction. As the free cheeks in these old Cambrian trilobites have been in a very loose connection with the fixed cheeks and generally deciduous, contrary to the condition in the Silurian ones, it is in many instances very difficult to tell whether species with facial ridge, especially those from the earlier Olenus schists have been blind or provided with eyes. The order of succession of the genera in the Swedish uppermost Cambrian, in the Olenid slates is, according to S. A. TULLBERG'S researches[27] at Andrarum in Scania as follows: