226 TRILOBITA 



CHAP. 



been disputed. But Emmerich, Salter, and M'Coy l 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 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, 2 and joining the genal angle so as to cut off the 

 genal spine. Loven 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 Jfarpes 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 



1 M'Coy, Synop. Sil. Foss. Ireland, 1846, p. 56, and Brit. Pal. Foss., 1851, 

 p. 146, pi. 1 E, fig. 16 ; Salter, Quart. Journ. Geol. Soc. iii., 1847, p. 251. 



2 Figures showing this suture are given by Oehlert, Bull. Soc. gtol. dc France 

 (3), xxiii., 1895, pi. 1, figs. 9, 12, 15. 



