44 PALEONTOLOGY 



sea-bed, like those on the Potsdam .sandstone of America, cal- 

 led " Froticlmites," subsequently to be described. 



Ordei- Trilobites. 



Char. — Trunk segments trilobcd ; sessile compound eyes in 

 most ; limbs aborted. 



The great family of Trilobites is entirely confined to the 1 

 palreozoic 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 antennas 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 recpiisite. Variations in the length of the cephalic and 

 caudal spines {e.g. in A saphus 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, 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 Trinuclcus (fig. 9, io) with its ornamental bor- 

 der, and IUcvnns (fig. 9, 7), in which the trilobation is less con- 



