In the figure 5 (FORD'S fig. 1.) there are the first signs of the pygidium coming and in the fig. 6 it is well developed, but the thorax is still non apparent. In the stage fig. 6 both pleura have increased in length and the compound one also in bulk. They project with spiny points beside the beginning pygidium and the anterior pleura have united across the first segment of the glabella through a narrow ridge, which seems to cut that segment in two. The sequence in the order of development or growth thus is first the head, then the pygidium and last the thorax. At least it is so in these the oldest of all trilobites. But in nearly all trilobites of which there are good data, the head is the part first developed. Evidently a large hiatus exists between the stages represented in figs. 6 and 7 (FORD'S figs. 2 & 3, 1877), in the latter of which the animal, though not adult, has had the thorax and pygidium added to the head. The modifications in the size of the pleura are the chief changes. The anterior pair is reduced and retired within the posterior border of the head forming a semilunar arch joined with the occipital ridge in an angular bend. The posterior pair is enlarged and its spine is by and by reduced (figs. 8, 9, 10) till it quite disappears and only the wide semicircular field between the first pleuron and the glabella is left behind as a remnant of its dilated body. It is to be remarked, that while in the plurality of the adult Olenellidæ all traces of the spiny projections of the second pleuron have vanished, they are still retained in the adult Holmia Kjerulfi, though not in the American Holmiæ, and thus give it at the same time a more ancient and a more larval stamp. The shallow groove along the back of its first pleuron indicates strongly its pleural nature, as the thoracic pleura commonly are divided through such grooves. The same peculiarity is also observable in several of the American Olenellidæ. It is much the same with the posterior pleuron, the pleural nature of which is revealed through its spine, that is homologous and identical with the spiny terminations of the thoracic pleura. We have thus through the remarkable finds of FORD and WALCOTT combined received an explanation of the morphological origin and nature of the facial ridge, the so called eye lobe and found that it has nothing whatever of the character of a visual organ. But it must be borne in mind, that these developmental changes are peculiar only to the Olenellidæ, the origin of the facial ridge in the later trilobites is, as we shall see, quite a different one. The Olenellidæ belong chiefly to the oldest of the Cambrian beds with trilobites, and none of them has as yet been found higher up.