SUBCLASS I TRILOBITA 697 



they appear absent altogether in some genera {Co')iocoryphe, Agnostus), and are 

 so imperfectly shown in others that for a long time they remained unrecognised 

 {Agraulos, Sao, Ellipsocephalus, 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 surface (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 Encrinurus and Trimerocephalus ; large and prominent, as in 

 Phacops, Dalmanites and Proetus ; 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 

 as of two kinds. In the first, the holochroal, the visual area is covered with a 

 continuous horny integument, or cornea, which is either smooth and externally 

 gives no idea of its compound nature, or granular, on account of the facets 

 beneath. The lenses of the ommatidia are often visible by translucence. The 

 second type of structure, the schizochroal, is confined to the single family 

 Phacopidae. In this, the visual area is made up of small, round or polygonal 

 openings for the separate facets of the cornea, between which is an interstitial 

 test or sclera. The size of the facets varies from more than 0'5 mm. in some 

 of the Phacopidae, to from 6-14 facets in the width of 1 mm. in other Trilobites. 

 The number and arrangement of the facets also vary greatly according to the 

 genus. Trimerocephalus volhorthi shows only 14 facets, while species of Phacops 

 may possess from 200-300, and Dalmanites hausmanni has 600. Among the 

 holochroal eyes, the number of facets is much greater ; in Goldius palifcr it is 

 estimated at 4000, in Ogygites nohilis at 12,000, and in Caphyra radians as high as 

 15,000. Usually the facets are arranged in regular, alternating, vertical rows, 

 or quincuncially. 



Certain genera show visual organs of an entirely different type, which can 

 be best regarded as simple eyes, and correlated with the ocelli of many Crus- 

 taceans. Thus, the genera Harpes and Tretaspis present from one to three 

 simple elevations or granules on the fixed cheeks, at the ends of eye-lines, 

 while the ordinary compound eyes on the free cheeks are absent. 



The Thorax. — In contrast to the undivided cranidium, the thorax consists 

 of a series of short, transverse, articulating segments, which difter in number 

 with the genus and species. Every thoracic segment is divided by the dorsal 

 furrows into a middle portion (axis, tergum) and two lateral divisions {pleura, 

 epimera). The axial portions are firmly anchylosed with the pleura, and are 

 generally strongly convex, with the posterior margin incurved. Anteriorly 

 they bear an extension below the general surface, and separated by a furrow. 

 This forms a surface of articulation along which the segments are movable, and 

 is covered by the edge of the segment immediately in front, so that it is chiefly 

 visible in coiled or disarticulated specimens. Barrande distinguished two 

 types of pleura : (1) furrowed pleura (j)lhre a sillon), which have a diagonal 

 furrow on the upper surface, running posteriorly from the anterior edge near 



