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). [Illustration: 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.