TRILOBITES.
Char.—Trunk segments trilobed ; sessile compound eyes in most ; limbs aborted.
The great family of Trilobites is entirely confined to the paheozoic age ; none are found even in the upper coal measures or Permian system. Above 400 species have been described, and grouped in 50 genera. Of these 46 are Silurian, 22 De vonian, and 4 carboniferous. According to Bronn, 13 genera are peculiarly Lower Silurian, 3 Upper Silurian, 1 Devonian, and 3 carboniferous.
The skeleton of the Trilobite consists of the cephalic shield, a variable number of trunk-rings or segments, and the pygidium or tail composed of a number of joints more or less anchylosed. In some species a labrum (or " hypostome") has been discovered, but no indications of antennae or limbs have ever been detected ; still there can be no doubt they enjoyed such locomotive power as even the limpet and chiton exhibit when requisite. Variations in the length of the cephalic and caudal spines (e.g. in Asaphus caudatus and longi-caudatus), and in the prominence of the head-lobes, have been considered indications of difference of sex. One of the oldest and simplest forms is the minute Agnostus (fig 9, ii) ; it is usually found in little shoals, with only the cephalic shield preserved, as if it were the larval form of some large Trilobite. According to the
observations of M. Barrande, the Sao passes through twenty stages of growth, being first a simple disc, and ultimately having seventeen free thoracic segments and two caudal joints ; the additional segments are developed between the thorax and abdomen. The Trinucleus (fig. 9, io) with its ornamental bor der, and Illcenus (fig. 9, 7), in which the trilobation is less con spicuous than in most genera, are characteristic of the Lower Silurian strata. Two others from the Wenlock limestone have long been celebrated, viz., Calymene (fig. 9, 9), or the " Dudley Trilobite," so compactly rolled up ; and Asaphus (or Phacops) caudatus (fig. 9, s), in which the lenses of the large eyes are frequently well preserved, and visible without a glass. Each eye has at least 400 facets, and in the great Asaphus tyrannus each is computed to have 6000. In one species (Asaphus Kowalewskii) the eyes are supported on peduncles. The largest Trilobite is Asaphus gigas ; some of the fragments indicate a creature eighteen inches long.