ARTHROPODA. TRILOBITA. 



199 



Arachnida, rather than to the Crustacea, it is possible that 

 the Trilobites will also have to be placed in that class. 



FIG. 54. Under surface of Asaphus megistos, from the Ordovician rocks 

 of Ohio, showing a pair of appendages attached to each segment. 

 The shaded part represents the actual specimen, a, infolded margin 

 of the head-shield; b, hypostome; c, branchial filaments? (After 

 Walcott.) 



They agree with Limulus in the trilobation of the body 

 and in the possession of a similar shield covering the 

 anterior part of the body; with the Palaeozoic Xiphosurans 



