(0 is new, added here to supplement fig. I.)

Brögger has also succeeded[24] in finding a series of small larva, which he considers as belonging to a species of Liostracus. As the figures drawn by Brögger twenty five years ago may now be very little known, I here reproduce them with the kind permission of the Editors of »Geologiska Föreningens Förhandlingar», where they were published in 1875. This development proceeds nearly upon the same plan as in Sao. The first stage, however, (I) seems to be much earlier than any of Sao, the rhachis or glabella consisting of an unsegmented ridge of more primitive appearance. Before this first stage of Brögger's larva a still older phase of development can be imagined, (0) a simple rounded, smooth head shield without any indication of a glabella at all. This stage might correspond to the head of certain species of Agnostus, as A. glandiformis, A. nudus, which have no glabella. This stage 0 is also valid for the larva of Sao. Several stages are evidently wanting between I and II, in the later of which the thickened glabella is divided in four segments. In III we have six segments, all these three stages consisting only of the ovate head with narrow fixed cheeks. In IV the pygidium has been added to the primitive head, but the segments of the glabella have been reduced to four and in V slightly altered in shape In VI, again, we see the head with five glabellar segments and scarcely the first sign of the facial ridge. Between VI and VII there must be links missing, as the change can not be so abrupt, and likewise between VIII and IX as in VIII there are still no free cheeks nor any facial ridge. This interesting discovery of Brögger confirms, together with those of several other authors[25], the supposition that the development of the later Cambrian and older Silurian forms is a quite 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]

[24] Fossiler fra Öxna og Klettna, Geol. För. Förhandl. 1875, p. 572, pl. 25, fig. I-X.

[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.

[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:

1. Parabolina (oldest division of the Olenus slates).
Olenus.
Liostracus.
2. Eurycare.
Leptoplastus.
3. Peltura.
Sphærophthalmus.
Ctenopyge.
4. Cyclognathus (uppermost).
Acerocare.