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, 7) 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. 7rinucleus, 
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, y)—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, ¢), 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 
elabella. 
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. Lindstrém 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 1s 
not accepted by Lindstrom. 
The existence of a facial suture in 77rinucleus has likewise 
VOL. IV Q 
