BUB-CLAS8 I 



TlMl.nlilTA 



to which they are 

 They may occupy 



/ . /.'. '', 



! ami fri'i,' 



(.ifl-r Nnvak). /;/;, Ant.M-i. Mi.hlle 



furrow; !:, Posterior furrow of tin- ini'l'll<- \>r- 



tion ; /', I'o.strrinr edf ; >J, Pot- 

 ti-riur win;,'. 1 



In front of the hypostoma is a rostral area sometimes partly ..,- -upird Ky 

 ;i separate plate. 



The fixed (-lurks are lateral e\ten>ion> from the ^laln-ll 

 firmly joined, forming tin- mitral portion of tin- rrphalon. 

 more than two-thirds of the cephalon, as 

 in Conocoryphe, or become greatly reduced, 

 as in Ann /thus, 7,//7/r/s, and J'r<-ln.<. The 

 '/n it/i// ii nt, consists of the glabella and tin- 

 fixed cheeks. 



The free cheeks carry the compound 

 eyes, and are separated from the cranidium 

 by a suture. They may form (a) a con- 

 tinuous ventral plate, as in 7A/ //.<. 

 ./V//W//S, TrinHclcus, etc.; they may in- 

 clude (b) a greater or lesser portion of 

 the dorsal surface, being either entirely separated by the cranidium, or 

 (c) meeting and (d) sometimes coalescing in front. They are widely separated 

 in Ptychopa/Ha, in juxtaposition in Asaphus, and continuous in Dalmanites. 



The iff mi I angles are the posterior lateral angles of the cephalon. They 

 may be rounded, as in Illaenus, angular, as in Bronteus, or spiniform, as in 

 Ti-i a in-lt' us and Dalmanites. They belong either to the fixed cheeks, as in 

 Dalmanites, or to the free cheeks, as in Illaenus, Bronteus, and Proetus. 



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 

 (Agraulus, Sao, Ellipsocephalns, etc.). 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 J^in-riiiiii-ini and T/-///H /<>,>/>//, ilns ; large and prominent. 

 Phacops, Dalmanites, and Proetus; or very large, as in ./"//V/<i/, 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 



