364 



ARTHROPODA 



abundant in the Silurian. The body (fig. 380) consists of head and trunk, 

 the latter segmented. In the young the segments are very few, but increase 

 in number (10-29, according to the species) with age. The hinder seg- 

 ments frequently differ from the rest and form an abdomen or pygidium. 

 Dorsally the animal is divided by two grooves into three lobes, marking 

 off in the head a glabella and two gcncr; in the trunk rhachis and two 

 pleurcc. On the head there are usually a pair of compound eyes, which 

 in the young were frequently ventral, but are brought to the dorsal surface 

 with growth. For years little was known of the ventral side, but lately 

 specimens of Triartlims bccki (fig. 381) from the Utica slate have revealed 



FIG. 380. FIG. 381. 



FIG. 380. Paradoxides bohemicus (from Zittel). 



FIG. 381. Triarthnis becki, ventral surface, restored (after Beecher). The head 

 bears one pair of antennae and four pairs of biramous feet, the basal joints serving as 

 maxillae. Trunk with biramous feet. 



* 



the appendages. On the head are a pair of simple antennae, and four 

 pairs of schizopodal feet, the bases of which acted as jaws. It is a 

 question whether the first pair of jaw feet correspond to the second 

 antennae or whether these have been lost in the group. The trunk 

 segments bear biramous feet. In some respects the trilobites resemble 

 the Xiphosura (infra), but the possession of antenna and biramous feet 

 place them among the Crustacea. Here their position is very uncertain. 

 We have little knowledge of but one species, and this with its single 

 pair of antennas differs from all recent Crustacea. 



