Group 1. The Olenellidæ.
In this group we have two families of different age, the older, less developed the Olenellidæ proper, and the younger the Paradoxidæ. The former consists of the genera Olenellus, Holmia, Mesonacis and Schmidtia, and we shall attend to them first. They have no facial suture[6] and consequently a tripartite head shield like the Archaic ones, no eyes, but there is that strongly developed and most characteristic facial ridge. As seen in Olenellus Thompsoni[7] the crescent like ridge starts as a direct outflow from the base of the first segment of the glabella, and is in direct continuation with it as an integral part. It is roundbacked, regularly curved and at its starting point as broad as one of the posterior glabellar segments. It is regularly semilunate, tapers posteriorly and ends near the occipital segment. From the narrow second segment of the glabella, inside the just described larger ridge, a smaller ridge emanates, broader and flatter than the former, slightly curved, and ends between the third and fourth glabellar segment. Already in Holmia Kjerulfi the ridge is modified. There it is only one ridge, the anterior one, nearly as doubled through a shallow groove running along its back. The second one has dwindled away so as to be seen only as a narrow stripe near the occipital segment and ending outside this in a point. The anterior ridge, represented by Holm as consisting of two nearly independent parts, is indeed in one piece, though its dorsal groove sometimes is deep, and it is with its total breadth joint to the first glabellar segment. Along its outer edge, where it lies close to the cheek, a narrow slit runs, and I suppose that it is to be considered as the first indication of the forming of a facial suture, which however does not reach longer than the ridge. As in Paradoxides it is so tight to the cheek, that there has been no place for an eye. It is no accidental break, its edges being too regular and unbroken in all specimens. This ridge is by far much shorter than in Olenellus and terminates opposite the third segment.
[6] Beecher Natural Classification of the Trilobites, p. 191, pretends that in Olenellus and Holmia real sutures »in a condition of symphysis» occur. He seems to deny the facial sutures and to accept as »real sutures» the »internal sutures» described by Holm in Olenellus Kjerulfi. It is highly doubtful if these interior elevated lines are to be regarded as sutures. They are indeed no sutures, but in reality elevated linear ridges, inclosing, as it were, narrow canals. The real sutures known, the facial sutures, never form elevated lines, be it on the outside of the head or on the inside. Probably these lines are derived from some now unknown interior organization and it may be fit to remind of the somewhat similar though more numerous linear canals of the branchiæ on the interior surface of the great head shield of Apus. (Zaddach De Apodis cancriformis Anatome ... pl. II fig. 1) or what Huxley (Anatomy Invertebr. Animals p. 281) calls the convoluted »shell gland» in the carapace. A quite different structure is what I suppose to be the incipient facial suture in Holmia.
[7] Walcott in 10th Ann. Rept. U. S. Geol. Survey, pl. 82 f. 2.
Larva of Olenellus asaphoides, chiefly according to
Ford and Walcott.
1. Hypothetical figure, based on fig. 2. represents a stage preceding the next. There are three pair of pleura and three segments of the rhachis. |
2. Copy of Walcott's fig. 5 in »Bullet. 1886», pl. XVII. Three pleura at right, two at left, the larger formed through fusion of the two hindmost in fig. 1. |
3. Copy of Walcott's fig. 6: the second and third pleura have been united into one. |
4. Hypothetical figure as a further development, following upon the stage represented in fig. 3. Fusion completed between all posterior plectra (2, 3, 4), thus forming a single large pleuron composed of the three mentioned. |
5. Copy of Ford's fig. 1, pl. IV, Amer. Journ. Science 1877, enlarged to the same size as Walcott's. First sign of the pygidium. |
6. Copy of Ford's fig. 2 (1877) enlarged. The pygidium has been added to the head shield. |
7. Copy of Ford's fig. 3, enlarged. | |
8. Copy of Ford's fig. 1 page 251 in Am. Journ. Oct. 1881, enlarged. |
9. Copy of Ford's fig. 2 page 251. Amer. Journ. Science 1881. Slightly enlarged. |
10. Copy of Ford's figure 5, pl. IV, Amer. Journ. Sci. 1877, slightly enlarged. | |
Already Linnarsson observed the second ridge as he tells in a paper where he describes Olen. Kjerulfi for the first time.[8] It has been called an »ornamental» spine, but in the following we shall learn what it really is. This ridge in connection with its spine has not been observed in any other of the Olenellidæ, at least not in their adult stage.
[8] Öfversigt Vet. Ak. Förhandl. 1871, tafl. XVI fig. 1.
Thanks to the discovery of very early larval stages of the American Olenellus asaphoides, which Ford[9] and Walcott[10] have described and figured, we can through combination of these decipher the development and signification of the facial ridges. To facilitate my explanation I here join a series of cuts from the earliest stage to the more developed, with addition of two schematic stages to complete in a certain degree the series of Walcott and Ford.[11] See figures [p. 13].
[9] In American Journ. of Science 1877 p. 265 and 1881 p. 250.