226 TRILOBITA CHAP, 
been disputed. “But Emmerich, Salter, and M‘Coy' have main- 
tained 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 7rinucleus 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 7’rinucleus. There is, however, 
in that genus a suture running close to the margin of the 
cephalic border,’ 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, how- 
ever, 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 Zrinucleus 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 
1M‘Coy, Synop. Sil. Foss. Ireland, 1846, p. 56, and Brit. Pal. Foss., 1851, 
p- 146, pl. 1 E, fig. 16; Salter, Quart. Jowrn. Geol. Soc. iii., 1847, p. 251. 
2 Figures showing this suture are given by Oehlert, Bull. Soc. géol. de France 
(3), xxili., 1895, pl. 1, figs. 9, 127Ml5. 
