TRIL'OBITES. an order of fossil crustacea entirely confined to the paleozoic rocks. They are specially abundant in the Silurian period, and disappear in the lower members of the coal-measures.
The body was covered with a chitinous shield, which consisted of a large united cephalic shield, a variable number of body segments, and a tail or pyrridiuni, composed of a number of joints, more or less ancliylosed. The eyes were sessile and compound. The lenses are frequently benutifully preserved, and in some species are so large that they can easily be seen with the naked eye. In asaphus eaudatus, each eye had at least 400 facets; and in the large A. tg•annus, it is estimated that there no fewer than 6,000. In some species, a bifurcated plate has been found in the region of the mouth, which is believed to be a lahrum, but no antenna or limbs have been yet detected in any specimen. They may have been entirely destitute of antennte, As in some living animals to which they are nearly related these organs are very rudimentary; and their feet were probably soft and leaf-like appendages, bearing the gills, which would weedily perish, and leave no traces in a fossil condition. The sexes are believed to be indicated by variations in the length of the cephalic and caudal spines. 'and in the prominence of the head lobes. The members of the order varied greatly in size, sonic
species being scarcely larger than a pin's head, while others, like ersapinis gigas, attained a length of 18 inches. It is probable that many named species may be only larval or transition forms of others. 'I he minute agnosteus is frequently found in such quantities as to indicate that it lived in shoals, as if it were the larval Form of some large trilobite. Burmeister considers that trilobites have their nearest allies in the minute phyllopeda, section of entomost•acons crustacea, which live in stagnant water, and are never at rest, but continually swimming, at various depths on their racks, some beimg so near the sur face of the water that their feet touch it. Ile corsequently supposes that trilobites lived gregariously in shallow water close to shore, moved only by swimming, near the surface, end could not creep at the bottom; that they swam iu an inverted position, with the belly upward; that they made use of their power of rolling themselves into a Lail as a defense against attacks from above; and that they lived on smaller water-animals.
Above 400 species have been described, and grouped into 50 genera. Of these, 46 arc Silurian, 22 Devonian, and 4 carboniferous.