612 ARTHEOPODA SUB-KINGDOM vn 



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 in the width of 1 mm. in other Trilobites. 

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

 genus. Trimerocephahis Volbortlii shows only 14 facets, while species of Pliac<>i>* 

 may possess 'from 200-300, and Dalinanites Hausmanni has 600. Among the 

 holochroal eyes, the number of facets is much greater ; in Bronteus palifer it is 

 estimated at 4000, in Asaphus nobilis, and in Eemopleurides 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 genus Harpes, and some species of Trinudeus 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 vary in number 

 with the genus. 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 arichylosed 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 : (a) furrowed pleura (plewe a sillon), which have a diagonal 

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

 the axis, and towards the free extremity ; and (b) ridged pleura (plewe a 

 bourrelet), having a longitudinal ridge or narrow fold. These characters vary 

 considerably, and are sometimes obscure. In a small number of genera 

 (Illaenus, Nileus) the pleura are perfectly smooth. 



All pleura show a distal or lateral, and a proximal or inner portion. The 

 latter extends from the axis to the fulcrum or bend, i.e. to a place where the 

 pleura bend more or less abruptly downward, and also generally toward the 

 rear. The distal portion, beginning at the fulcrum, may continue of equal 

 thickness and be rounded or obtuse at the extremity, or it may decrease in 

 size and terminate in a spine. 



The number of thoracic segments varies exceedingly among different 

 genera. The smallest number (2) occurs in Agnostus. The largest number 

 so far observed (29) is found in some species of Harpes. A variation is to be 

 noted even among the species of a single genus, hence this character is not of 

 general application for purposes of classification. For example, there are 

 species of Ampyx and Aeglina with five to six thoracic segments, Phillipsia with 

 nine to fifteen, Cheirurus with ten to twelve, Cyphaspis with ten to seventeen, 



