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.