FIG. 138.—_Phacops latifrons_, Bronn, × 1. Devonian. Showing large compound eye. (After Zittel.) ] A pair of compound =eyes= are present in the majority of Trilobites. Each eye is situated on the free cheek, at that part of its inner margin where the facial suture bends to form an angle (Figs. 137, A, _h_, 138). The position of the eye is consequently determined by the position of the facial suture; it may be near the glabella or near the lateral margin of the head, and either as far forward as the first segment of the glabella or nearly as far back as the neck-furrow. In many Trilobites the eye is more or less conical, with its summit truncated or rounded, but in some genera it is ovoid, or crescentic. In _Aeglina_ (Fig. 150, H) the eye is flattened and scarcely raised above the general level of the cheek. The eye of a Trilobite is oriented so that its longer axis is parallel or nearly parallel to the axis of the body (Fig. 150, G); but in one case (_Encrinurus intercostatus_) it is placed at right angles to this axis. The size of the eye varies considerably; it is largest in _Aeglina_, in which it covers nearly the whole of the free cheek; it is small in _Acidaspis_ and _Encrinurus_. Though the eye is always entirely on the free cheek, the adjoining part of the fixed cheek is raised to form a buttress on which the eye rests; this buttress, which is known as the “palpebral lobe,” is seen clearly when the fixed cheek is removed. The eyes of Trilobites are always sessile; for although in some species, such as _Asaphus cornigerus_, _A. kowalewskii_, and _Encrinurus punctatus_, they are on the summits of prominent stalks, yet those stalks are immovable. Three types of compound eye have been recognised in Trilobites[184]— holochroal, prismatic, and schizochroal. [Illustration: FIG. 139.—Eyes of Trilobites. (After Lindström.) =A=, =B=, _Sphaerophthalmus alatus_, Ang. Upper Cambrian. Vertical and horizontal sections, × 100. =C=, _Asaphus fallax_, Dalm. Horizontal section, × 60. =D=, _Nileus armadillo_, Dalm. Vertical section, × 60, _a_, prismatic lenses; _b_, cuticle; _c_, part of free cheek. =E=, _Dalmanites vulgaris_, Salt. Part of eye, × 30. =F=, _Dalmanites imbricatulus_, Ang. Vertical section of eye, with a part of the free cheek on the left, × 60. =G=, =H=, _Harpes vittatus_, Barr. =G=, The two lenses of one eye, × 8; =H=, vertical section of the same, × 60. ] 1. In the holochroal eye (Fig. 139, A, B) the lenses are globular or biconvex and close together, so that the cornea is continuous over the entire eye. Examples of this are seen in _Bronteus_ and _Sphaerophthalmus_.