612 ARTHROPODA SUB-KINGDOM VII 
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. T'rimerocephalus Volborthi 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 Bronteus palifer it is 
estimated at.4000, in Asaphus nobilis, and in Remopleurides 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 7’rinucleus 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 Thoraz.—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 (aais, tergun) 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: (a) furrowed pleura (plevre & 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 ()) ridged pleura (plevre @ 
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 fulerum or bend, 7.c. to a place where the 
pleura bend more or less abruptly downward, and also generally toward the 
rear, The distal portion, beginning at the fulerum, 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, 
