VIII HEAD 225 



anterior lobe of the glabella by fulcra which are developed for 

 the attachment of muscles. 



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 baokwardly 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. Trimicleus, 

 Fig. 140, B; Jfarpes, 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 " (/). 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 (6) 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 con- 

 sequently the free cheek, if present, must be on the ventral 

 surface. Lindstrom 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 Lindstrom. 



The existence of a facial suture in Trinucleus has likewise 



VOL. IV Q 



