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.