262 EMMERICH ON THE MORPHOLOGY, CLASSIFICATION 
this marginal suture is pretty nearly uniform in all. The suture: 
between the forehead and maxillary parts, the facial suture 
(linea s. sutura facialis), on the contrary, is of very considerable 
importance. The position of the eyes being invariably close to 
the facial suture, and indeed in such a manner that the latter 
separates the real field of vision of the eye from the forehead 
portion, it enables us to describe accurately the course of the 
suture. This may naturally be divided into two parts; the one 
in front of the eye, and the other behind it: at the eye itself it 
generally circumscribes a particular lobe of the forehead portion, 
which covers the upper part of the ocular cone (the upper eyelid 
of Dalman), and which therefore enables us to determine the 
position of the eyes, even in separated maxillary parts and eyes. 
This so-called eyelid is absent in only very few Trilobites (Para- 
doxides). The course of the facial suture in front of the eye fre- 
quently exhibits remarkable differences even in species which 
are nearly related to one another (Asaphus extenuatus, Dalm., 
and As. expansus, Wahlenb.), and is therefore of little conse- 
quence ; but it is very characteristic for several genera, that the 
forehead part becomes so broad, and the cheeks proportionately 
so narrow, that the apices of the latter do not touch, as usual, 
in front of the forehead; the facial lines of both sides do not 
meet in that case, but terminate in the marginal suture. The 
facial suture and the cheeks might be separately called in this 
case gene s. linee faciales discrete. This occurs in Ellipso- 
cephalus Hoffii, in the most distinct manner ; it may also be seen 
however in Amphion, Illenus, and in some of the Asaphi, also 
in Calymene Blumenbachii. The direction of the suture behind 
the eye, on the other hand, presents very important differences. 
In Jilenus it proceeds in a tolerably straight direction towards 
the posterior margin; more frequently it runs at first pretty 
parallel for a shorter or greater distance with the posterior mar- 
gin of the cephalic shield, and then terminates at the external 
margin; it joins this latter, sometimes forming a right angle, 
sometimes under a more or less acute angle, from its changing 
its direction towards the end, and running backwards for a, ge- 
nerally, short distance. This is the course in Phacops. Inter- 
mediate between Phacops and Illenus are the genera Calymene, | 
Brong.,emend., Homalonotus and Conocephalus, in which the facial 
line terminates in the two angles of the cephalic shield. Its 
most frequent course however is that of a curve, the convexity 
