[10] Bullet. U. S. Geol. Survey N:o 30, 1886, pl. 17 fig. 5-6.—Tenth Annual Rept. U. S. Geol. Survey, 1888-89, printed 1890, plate 88, fig. 1, 1 a.
[11] In his paper on »The larval stages of Trilobites» p. 175 Beecher gives a new figure (f. 6), original from Ford's collection, of the larva of Ol. asaphoides, but it is so sketchy that I cannot with certainty make out what it means. I cannot agree with him when he speaks of free cheeks and eyes in these and he is completely wrong when he says that the outer pair of spines belong to the free cheeks etc. (p. 176).
The [figure 1] has been hypothetically composed as a deduction from [fig. 2], which presupposes an earlier stage of development like that in [fig. 1], when there existed three or four pair of lateral appendages in the larva. This then consists of a central portion of five segments. The large anterior crescentlike segment does not, however, show any distinction between a central part and lateral appendages, it is nearly as large as the three next taken together and its backwards bent side horns embrace the next two posterior segments and attain with their narrow pointed tips the back of the fourth segment. The central portion consists of five segments, when the somewhat not well definite posterior marginal segment is taken in account. Each of these segments excepting the fifth one has lateral appendages, those of the second and third segment being quite as broad as the central part and bent backwards in a curve ending in a small pointed spine. The lateral appendages of the fourth segment are largest of all, more than double the length of the two next in front, triangular and standing out beyond the posterior border of the shell as a broad spine.
In [fig. 2] a great change has set in. There is no distinction between the lateral appendages of the third and fourth segments at left. These two have been fused together, they have united, so as to make the left triangular spine look larger than it was originally. The appendages of the right side are still in the same state as before. But this fusion of the lateral appendages also takes place in another direction, as shown in another specimen ([fig. 3]), so that the second and third appendages on both sides coalesce into one piece. Now it is easy to imagine that at last a complete fusion has set in between all lateral appendages and that instead of the original three on each side, there is only one large piece, reaching beyond the shell as a broad spine, as represented in the hypothetical [figure 4].
In the [figure 3] a progressive change is also seen in the transformation of the first central segment. From occupying the whole foremost space of the shell it has been lessened in size, more distantiated from the anterior border of the shell and rounded off, more prominent and definite from the lateral appendages, which have become narrow, though of the same length as before. The two first central segments seem to have been united into one.
Between the hypothetical [figure 4] and [fig. 5], there is evidently a great lacuna, not yet filled up. In the interval of time the two appendages, which we saw in [fig. 4], have been much modified, the posterior one having lost so much in bulk, and the anterior being lengthened and stretching out beyond the border of the shell in a narrow spine alongside the posterior one. The central segmented part has now assumed a shape, which on comparing it with the following stages of development makes its true nature evident and that it indeed is what in the adult animal becomes the glabella. The meaning of the previous stages then also is easily understood. The central segmental piece in them is the glabella or we may, as Bernard already has done, call it for rhachis and the side appendages for pleura, as this little larva represents the whole body of the future trilobite, and embodies all its parts in nuce. Through the great changes which these pleura undergo, it results, as we have seen, that two pair vanish, being incorporated with the large fourth pair and that only two rest for a while, the anterior one being the so much renowned frontal-lob or eye lobe and the second one the so called »ornamental spine», which in fact is a compound of the original second third and fourth pleura of the corresponding segments.
It is to be borne in mind that this larva, which represents the future trilobite in its earliest stage, is nothing but the head, or what in the adult takes the place of the head, and especially its dorsal surface and that it thus solely consists of the future head.
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.