When the glabella reaches the front border of the head the two cheeks are separated (Fig. 150, I); but in other cases they unite in front of the glabella (Fig. 150, C). The outer posterior angle of the cheeks or genae (“genal angle,” Fig. 137, A, _i_) may be rounded, pointed, or produced into backwardly directed spines (Fig. 140). The marginal part of the cephalic shield is often flattened or concave; this border may be quite a narrow rim as in _Calymene_ (Fig. 137, A), but in some genera (e.g. _Trinucleus_, Fig. 140, B; _Harpes_, Fig. 150, A; _Asaphus_) it attains a great development. Each cheek is usually divided by a suture— the “facial suture” (Fig. 137, A, _g_)—into an inner and an outer part; the former is the “fixed cheek” (_e_), and the latter the “free cheek” (_f_). The course of the facial suture varies in different genera: on the posterior part of the head it begins either at the posterior margin (Fig. 150, C) or at the posterior part of the lateral margin (Fig. 151, C, D); at first it is directed inwards, and then bends forward, forming an angle. In front it may (_a_) end at the front margin (Fig. 147), or (_b_) be united beneath the front margin by a rostral suture (Fig. 137, B, _d_, D, _e_), or (_c_) unite with the other suture on the dorsal surface in front of the glabella (Fig. 151, C). In the last case the free cheeks also unite in front of the glabella. The facial suture is one of the distinguishing features of the Trilobites, and may have been of some use in ecdysis. In only a few forms is it absent, as for example in _Agnostus_ (Fig. 146) and _Microdiscus_. In the former, however, Beecher states that a suture is really present, but, unlike that of most other Trilobites, it is situated at the margin of the cephalic shield, and consequently the free cheek, if present, must be on the ventral surface. Lindström and Holm, after a re-examination of well-preserved specimens, deny the existence of a suture in _Agnostus_. By most authors _Olenellus_ is said to be without a suture, but Beecher maintains that although the fixed and free cheeks have coalesced, yet a raised line passing from the eye-lobe to the posterior margin marks the position of the suture; this view is not accepted by Lindström.