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48 PALAEONTOLOGY. 



Order Trilobites. 



Char. — Trunk segments trilobed ; sessile compound eyes in 

 most ; limbs aborted. 



The great family of Trilobites is entirely confined to the 

 palaeozoic age ; none are found even in the upper coal measures 

 / 6 c or Permian system. Above 400 species have been described, 



and grouped inj>0 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 ccmdatus and A s. 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 Agnostics (fig. 9, n) ; 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 Illeenus (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 



