These three types of eye, according to Lindström, have appeared successively in chronological order: the prismatic occurring first in the Olenus beds (Upper Cambrian), the holochroal first in the Ceratopyge Limestone (Uppermost Cambrian), and the schizochroal first in the Ordovician. The number of lenses in the eye varies greatly. For example, in Trimerocephalus volborthi there are 14 only, whilst in Remopleurides radians there are as many as 15,000. Even in different species of the same genus there may be considerable differences. Thus Bronteus brongniarti possesses 1000, B. palifer 4000, lenses in each eye. The number increases from the young up to the adult, but decreases in old age. The lenses are usually arranged in alternating rows. In Trilobites with a conical eye the outer segment of the cone bears the visual surface. It has been stated that the eyes of Trilobites resemble those of Isopods,[[186]] but close comparison is difficult to make, since in Trilobites no part of the eye beneath the lenses is preserved. In some genera a threadlike ridge, called the “eye-line,” passes from the glabella, generally from the front segment, to the eye, where it often ends in the palpebral lobe; this eye-line is found in nearly all genera which are confined to the Cambrian period, and persists in a few of later date, as for example in Triarthrus, Euloma, and some species of Calymene from the Ordovician; in Arethusina and Acidaspis from the Silurian; and in Harpes from the Devonian (Fig. [150], A).
Fig. [140].—Trinucleidae. A, Orometopus elatifrons, Ang. × 5. Restoration based on specimens from the Upper Cambrian (Tremadoc) of Shineton, Shropshire. B, Trinucleus bucklandi, Barr. Ordovician, Bohemia. A complete but not fully-grown individual showing eyes. Natural size. (After Barrande.) C, Ampyx rouaulti, Barr. × 3. Ordovician, Bohemia. (After Barrande.)
In Harpes and in some species of Trinucleus eyes are present, but have been stated to be of a different type. They are described as simple eyes, and have been compared with ocelli; they are never found in Trilobites which possess the compound eyes described above. In Harpes (Fig. [150], A) the eye is near the middle of the cheek, in the position where compound eyes occur in other genera; it appears to consist of two or three granules or tubercles which are really lenses, and is connected with the front of the glabella by an eye-line. No facial suture can be seen, consequently the whole of the cheek is stated to be the fixed cheek.[[187]] In Trinucleus (Fig. [140], B) a single tubercle is found on the middle of the cheek in the young of some species, and is sometimes connected with the glabella by an eye-line; the latter disappears before the adult state is reached, and in some species the tubercle also disappears, but in others (such as T. seticornis, T. bucklandi) it persists in the adult individuals.
From the lateral position of these eyes they can hardly be compared with the median simple eye of other Crustacea. In Harpes it is more probable that, as suggested by J. M. Clarke, they are schizochroal eyes imperfectly developed. Their structure (Fig. [139], G, H) is somewhat similar to that of schizochroal eyes, and moreover, in one species, H. macrocephalus,[[188]] there are, in addition to the three main tubercles, other smaller tubercles in regular rows. Further, the eye-line occupies the same position as in other Trilobites which have undoubted compound eyes. The absence of a facial suture cannot be taken as evidence against these eyes being of the ordinary type, since in some species of Acidaspis (e.g. A. verneuili, A. vesiculosa) which possess compound eyes there is, in consequence of the coalescence of the fixed and free cheeks, no suture.
In some species of Trinucleus (Fig. [140], B) the simple eye is found in the same position as the eye in Harpes, and if, as some writers have maintained, there is evidence of the existence of a suture in that genus, then there is no reason for regarding the eye as other than a degenerate form of compound eye. The probability of its being such is supported by the existence of a compound eye in a similar position in the allied form Orometopus (Fig. [140], A) which possesses a facial suture.
In some species of Trinucleus (Fig. [140], B) and Ampyx there is a small median tubercle on the front part of the glabella, which from its position may be a simple unpaired eye, but its structure appears to be unknown.
Some Trilobites possess no eyes. Well-known examples of such are Agnostus, Microdiscus, Ampyx, Conocoryphe, and some species of Illaenus and Trinucleus; such blind Trilobites are almost confined to the Cambrian and Ordovician periods. All the forms of later periods, with the exception of a species of Ampyx, and possibly one or two other species, possess eyes. In addition to those undoubtedly blind forms Lindström considers that most of the Olenidae and Paradoxidae were without eyes. Many of the members of these families possess a lobe closely resembling a palpebral lobe, and a corresponding excavation in the free cheek; such forms have been generally regarded as possessing eyes; and the absence of any indication of lenses in those cases, on which Lindström lays stress, has been explained by the comparatively imperfect preservation of these early Trilobites. The development of the supposed eye-lobe in some of the Paradoxidae and Olenidae differs from that of the eyes in other families of Trilobites. In the latter the eye appears first at the margin of the head and always in connexion with the facial suture. But in Olenellus, in which there is said to be no facial suture, development shows that the crescentic eye-like lobe (Fig. [145], E) is really of the nature of a pleura coming off from the base of the first segment of the glabella. In Paradoxides, which resembles Olenellus in many respects, a facial suture is present and forms the outer boundary of the eye-like lobe, but it is developed subsequently to the appearance of the latter, which seems to be similar to that of Olenellus. In some genera of the Olenidae the eye-line, which comes off from the first segment of the glabella, ends in some cases in a swelling or knob which has hitherto been regarded as a palpebral lobe, but according to Lindström’s view no trace of an eye has been found in connexion with that lobe, nor is there any space between the lobe and the free cheek in which the eye could have occurred. If this view is correct it follows that the majority of the Cambrian Trilobites were blind. The earliest genus with eyes would then be Eurycare found in the Olenus beds of the Upper Cambrian. Sphaerophthalmus and Ctenopyge, found in the higher beds of the Cambrian, also possessed eyes, but Olenus and Parabolina were probably blind.
On the ventral surface of the head there is a flat rim around the margin (Fig. [137], B, b); this rim or “doublure” is the reflexed border of the cephalic shield. In many Trilobites its median part in front is cut off by sutures so as to form a separate plate (e); such is the case when the two facial sutures (c, c′) cut the anterior margin of the cephalic shield and are continued across the doublure, where they are joined by a transverse or rostral suture (d) just below the margin. When, however, as in Phacops and Remopleurides, the two facial sutures unite on the dorsal surface, in front of the glabella, the median part of the doublure is not separated from the lateral parts, or from the dorsal part of the cephalic shield.
