SUB-CLASS I TRILOBITA 611 
In front of the hypostoma is a rostral area sometimes partly occupied by 
a separate plate. 
The fixed cheeks are lateral extensions from the glabella, to which they are 
firmly joined, forming the central portion of the cephalon. They may occupy 
more than two-thirds of the cephalon, as 
in Conocoryphe, or become greatly reduced, 
as in Asaphus, Lichas, and Proctus. The 
A. B C 

8 
cranidium consists of the glabella and the 
fixed cheeks. J 
The free cheeks carry the compound 
Fic. 1262. 
eyes, and are separated from the cranidium 
/ Hypostomas. 4, Lichas (Arges) palmata. B, C, 
by a suture. They may form (@) a con- Enerinurus intercostatus, side and front views 
tinuous ventral plate, as in Harpes, (ter Sopskh. bi Aviano wage: 3h, aie 
Agnostus, Trinucleus, ete.; they may in- OL HE BEC oslog es AME CMMs th Boe 
clude ()) a greater or lesser portion of = = 
the dorsal surface, being either entirely separated by the cranidium, or 
(c) meeting and (/) sometimes coalescing in front. They are widely separated 
in Ptychoparia, in juxtaposition in Asaphus, and continuous in Dalmanites. 
The genal angles are the posterior lateral angles of the cephalon. They 
may be rounded, as in IJl/acnus, angular, as in bronteus, or spiniform, as in 
Trinucleus and Dalmanites. They belong either to the fixed cheeks, as in 
Dalmanites, or to the free cheeks, as in J//aenus, Bronteus, and Proétus. 
The character of the cheeks is especially influenced by the facial 
sutures separating the free cheeks from the rest of the cephalon. They 
appear as sharply defined lines beginning either at the posterior margin, or 
near the genal angles, or on the lateral margins, and extend to the eyes, 
thence around the inner margin of the visual areas, then turn anteriorly, and 
either unite in passing around the front of the glabella or remain separate, in 
which case the sutures terminate in the anterior margin. ‘The position of the 
facial sutures thus determines the relative size of the fixed and free cheeks. 
After the death of the animal, or after moulting, the cephalic shield frequently 
fell into pieces, dividing along these sutures. 
In most Trilobites, the existence of eyes has been demonstrated, though 
they appear absent altogether in some genera (Conocoryphe, Agnostus), and are 
so imperfectly shown in others that for a long time they remained unrecognised 
(dgraulus, Sao, Ellipsocephalus, ete.). The eyes are compound, and are elevated 
above the free cheeks. The adjoining area of the fixed cheeks is also drawn 
upwards, thus forming the palpebral lobe. The visual areas of the eyes are 
borne by the free cheeks. The shape of this area is extremely variable, but 
together with the palpebral lobe it generally forms a truncated, conical, or 
semilunar elevation, of which the laterally directed, convex side is occupied 
by the visual area (Phacops, Asaphus). It may likewise have a circular or oval 
form, and very little convexity above the general surface. The eyes may be 
quite small, as in Enecrinurus and Trimerocephalus ; large and prominent, as in 
Phacops, Dalmanites, and Proétus ; or very large, as in Aeglina, in some species 
of which nearly the entire area of the free cheeks is faceted, and the visual 
surface extends around the entire outer borders of the cephalon. In many of 
the primitive genera the eyes are situated at the distal ends of raised lines, or 
eye lines, extending outward from near the forward end of the glabella. 
As regards their structure, the compound eyes of Trilobites are recognised 
