302 CRUSTACEA. TRILOBITA 



relatively large and the fixed cheek narrow. Beecher 

 considers that the protaspis of Trilobites corresponds to 

 one of the nauplius stages (the metanauplius) of recent 

 Crustacea, but that view is not accepted by Kingsley. 



The possession of antennae, the biramous character of 

 the other appendages, and the presence of five cephalic 

 segments, show that the Trilobites belong to the Crustacea. 

 The great variability in the number of segments in the 

 thorax and pygidium, the leaf-like character of the ap- 

 pendages on the posterior part of the body, the large hypo- 

 stome, and the gnathobases on the thoracic appendages 

 seem to indicate that the Trilobites are related to the 

 Phyllopod division of the Branchiopoda (p. 312), and es- 

 pecially to Apus and Branchipus. But the Trilobites differ 

 from the Phyllopods in the trilobation of the body, in the 

 occurrence of a facial suture, in the posterior segments 

 being fused together to form a pygidium, and in the 

 absence of a caudal fork. In the character of their append- 

 ages the Trilobites are more primitive than any other 

 Crustacea, since all except the first pair are very similar 

 in structure and show but little specialisation in different 

 regions of the body, and all are deeply biramous. Other 

 primitive characters are seen in the indication of segmen- 

 tation on the dorsal surface of the head, and in the presence 

 of a pair of appendages on every segment of the body except 

 the last. The Trilobites differ from most other Crustacea 

 in having only one pair of antenniform appendages. 



The Trilobites show some resemblance to the Xiphosura 

 (p. 338), and before their appendages were known, they 

 were thought by some writers to be allied to that group. 

 But they are clearly separated from the Xiphosura by the 

 presence of five cephalic segments, the biramous character 

 of the appendages, the occurrence of antennae and a 



