The existence of a facial suture in _Trinucleus_ has likewise been disputed. But Emmerich, Salter, and M‘Coy[181] have maintained that a suture is present in a normal position on the dorsal surface, extending from the posterior margin just within the genal angle to the eye (when present), and from thence bending forward and ending on the front margin near the glabella. It must be admitted that no indications of the suture are seen in the majority of specimens, perhaps owing to the fact that most examples of _Trinucleus_ are in the form of internal casts; perhaps also to the more or less complete coalescence of the fixed and free cheeks, since in no specimen has the free cheek been found separated from the rest of the head, as occurs not uncommonly in many other Trilobites. The probability of the existence of a suture receives some support from the fact that one is found in the allied genera _Orometopus_ and _Ampyx_ (Fig. 140). Barrande and Oehlert deny its existence in _Trinucleus_. There is, however, in that genus a suture running close to the margin of the cephalic border,[182] and joining the genal angle so as to cut off the genal spine. Lovén and Oehlert claim that this suture represents the facial suture, but in an abnormal position; this view, however, is not accepted by Beyrich. In this connection it should be noted that in _Acidaspis_, whilst the majority of the species possess a facial suture, there are two in which it has disappeared owing to the fusion of the fixed and free cheeks. Such being the case, it seems not improbable that the curved line passing backwards from the eye in _Harpes_ may mark the position of the suture; but it is stated that the only suture present in that form runs at the margin of the cephalic border, and is similar to that of _Trinucleus_. This matter will be referred to again when discussing the nature of the eyes in _Trinucleus_ and _Harpes_. The relative sizes of the fixed and free cheeks obviously depend on the position of the facial suture; when this starts on the lateral margin of the cephalic shield and passes forward to the outer part of the front margin, the free cheek will be a narrow strip; when, on the other hand, the suture starts from the posterior margin and runs close to the glabella, the free cheek will be relatively large and the fixed cheek narrow. The fixed cheek is small in _Phacops_, _Cheirurus_, and _Illaenus_; relatively large in _Remopleurides_, _Phillipsia_, and _Stygina_. It was suggested by M‘Coy[183] that the free cheek represents the pleura of an anterior segment which has not become fused with the other cephalic pleurae. The fixed cheek appears to be formed of the coalesced pleurae of the other cephalic segments, but of those pleurae the only indication seen in adult specimens is in the neck-ring; in young specimens of _Olenellus_, however, the presence of other pleurae is indicated by furrows on the cheeks in front of the neck-furrow. [Illustration: