Part 3

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. 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.

Thanks to the discovery of very early larval stages of the American Olenellus asaphoides, which FORD and WALCOTT 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. See figures p. 13.

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.

Next in order of evolution we have the important tribe of the Paradoxidæ. These are preeminently distinguished from the Olenellidæ through the well developed facial suture, which without exception in them all runs outside the facial ridge and separates this from the free cheek. This is a great step forwards in the evolution and establishes the fact, demonstrable also by other evidence, that the formation of the facial suture is subsequent to the appearance of the facial ridge. This preexisting ridge seems to have had no small influence on the development of the suture, it checked its progress from the front or from the sides toward the fixed cheeks and directed its course against the genal angles. It lay as a protection for the glabella against this disseverance, causing the separating line to run along its outside.

This group consists of the genus Paradoxides proper, as well as of Centropleura, Metadoxides and Hydrocephalus, if this is an adult form and not the larva of an unknown Paradoxides. Perhaps such forms as »Conocephalites» Emmrichi BARRANDE, as well as Anomocare limbatum, An. excavatum, Bathyuriscus and Dolichometopus may on account of the shape of their facial ridges be considered as related to the Paradoxidæ. But this must be left for coming researches to decide. Some American Cambrian forms also share in this characteristic and may upon closer inspection be ranged here. So Zacanthoides. In these as in the true Paradoxidæ the facial suture follows the ridge along its whole length, while in the trilobites of the third group the facial suture is in contact only with the posterior end of the ridge, the so called eye lobe. Remopleurides does not show characters, that as BEECHER thinks, could unite it with the Paradoxidæ. These are blind and Remopleurides has well developed eyes and an organization that gives it an isolated position in the system.

The facial ridge continues in a great variety of shapes, short or long, but always forming the fraction of a circle, always of nearly equal thickness, only slightly tapering towards one of the extremities, and always when in direct connection with the glabella, starting from the base of its foremost, largest segment. As a rule the ridge is more developed in the young or larval individuals, continuing from the glabella to near the posterior cephalic border in an uninterrupted arch quite as in several of the adult Olenellidæ of the oldest Cambrian. It can be taken as granted that its origin is the same as in the Olenellidæ though at present the only evidence at hand is the small larva of Paradoxides oelandicus, which LINNARSSON called Parad. aculeatus. In this we see the anterior pleuron or the facial ridge alone present, elongated downwards like the same pleuron in the figure 6 of Olenellus asaphoides and terminating like this in a fine spine stretching backwards outside the posterior border. Of the second pair of pleura there is nothing to be seen. This must then have been aborted at an earlier stage than in the Olenellidæ.

The connexion between the free cheeks and the middle part of the head has been very lax not only in the Paradoxidæ, but on the whole in nearly all Cambrian trilobites with free cheeks. When the free cheek is dissevered it shows no trace of the ridge, there is only a large scallop on the spot where it embraced the ridge. In the Paradoxidæ the rim of the indenture and the ridge are in so close contact that there is not the least place for an eye between them, as can be seen in the few specimens with a complete head. In all oculate trilobites again without any exception the facial suture separates that part of the eye which is the real visual organ with corneal facets, from the interior often elevated portion, opposite it, the so called palpebral lobe. The eye is always placed on the free cheek, the lobe again always on the fixed cheek of the head shield. No real eye exists without the palpebral lobe, and, on the other hand, that part of the facial ridge which later in the development changes to a palpebral lobe, occurs alone without any eye in a great number of Cambrian trilobites, and consequently these are blind and such is the case with the Paradoxidæ and a great number of the succeeding.

There is not the least evidence to support the suggestion that the »ocular ridge» is homologous, with the eye of Apus and that the real crystal cones lay sunk beneath the surface in a »water sac». As we, for instance, in Peltura have an »ocular ridge» (= facial ridge mihi) on the fixed cheek and opposite its posterior extremity, the »eye lobe», a real eye with facets on the free cheek it is not likely to suppose that the »ocular ridge» nor the »eye lobe» ever functioned as a visual organ or that two widely different sorts of eyes were placed in closest vicinity opposite each other.

It may be worth while here to remind of the great similarities, whether analogous or homologous, in the formation of the superior surface of the head in the trilobites and the embryons and the newly hatched larva of Limulus. The latter have the head shield separated into five portions, partly through a facial suture which, as in Peltura, Dalmanites and others, forms a continuous line around the glabella. This suture divides the eye node in a similar way as in the trilobites, that is, sectioning it in two parts, of which one adheres to the central fixed cheeks and the other to the free cheeks. And to judge by the figures of KINGSLEY the former, the whitish moiety is the first developed and sometimes for a while quite alone as the facial ridge of the trilobites and probably also anterior to the suture, as this is not complete at this stage. This white node reminds of the small facial ridge in Arionellus ceticephalus BARR.

DOHRN, Zur Embryologie and Morphologie des Limulus Polyphemus. Jenaische Zeitschrift 1871. A very good figure (pl. XIV, f. 4) shows clearly the two parts of the eyes, the interior one being larger.

KINGSLEY also (Devel. of Limulus, Journal of Morphology, vol. 7, 1892, pl. VI, fig. 34) has in the last larval stage the suture and the eye in two parts, of which one is white lying inside the suture and the eye proper, black, outside.]

As KISHINOUYA has already pointed out and as I have anticipated above the head of the most developed trilobites in their adult state, and the head of the larval Limulus consists of five parts, viz. 1) the glabella in the centre, 2 & 3) the fixed cheeks, 4 & 5) the free cheeks. An elevated ridge in the adult Limulus shows where the suture once lay and it is on the outside thereof that the eye of the adult is placed. What other authors call the ridge or the eye ridge in Limulus, KISHINOUYA rightly names a suture.

It was the renowned Swedish naturalist WAHLENBERG, who first recognized the importance of the facial suture, which he called »linea ocularis», but to another Swedish palæontologist DALMAN the exact definition of this suture is due, to which he gave the name still in use. He expressly remarks that the suture crosses the ocular node and limits the outside of the »lobus palpebralis» and he makes a clear distinction between that lobe and the »tuberculi and eminentiæ oculares» (= facial ridge) of which he says that they are the more or less evident elevations situated in the blind Palæades on the place of the eyes (in which he is wrong) »and which perhaps are an indication of such organs». But then he says doubtfully (p. 255) »tuberculorum ocularium veram naturam determinare haud ausi sumus, etsi oculorum formam sat bene exhibere videantur» and he adds concerning Paradoxides »oculi nulli, eorum loco autem tuberculi duo».

Group 2. The Olenidæ and related families.

Next in the ascending order we have the largest group of trilobites in the Cambrian, of which the greatest part is formed by the Olenidæ. A facial ridge different in shape and different in development from that in the former group characterizes them. Unlike the semilunate ridge of the Olenellidæ it issues mostly from the front of the first segment of the glabella and goes generally backwards till it meets the facial suture. It is narrow and fine as a thread, but for the rest assumes a great variety of forms. It may be curved as a circle segment as in Sao, Liostracus, it may be long and straight, standing out in a right angle to the glabella as in Eurycare, it may be short and straight in an acute angle to the glabella as in Parabolina and so forth. In the same genus, as for instance Olenus, straight and curved ridges occur in the various species, and consequently the form of this sort of ridge cannot be used as a generic character. It moreover differs from the ridge in the Olenellidæ and the Paradoxidæ in having the posterior extremity widened as a tubercle or node, which commonly has been called the »palpebral lobe», while in the older groups the ridge is only at the most a little pointed in both ends or of equal thickness.