198 ARTHROPODA. TRILOBITA. 



curvature occurs is known as the fulcrum ; generally each 

 pleura overlaps the anterior part of the succeeding one, 

 and often the fulcrum is rounded so as to form an articu- 

 lating facet. The terminations of the pleurae are in some 

 cases rounded, in others produced into spines. 



The pygidium (fig. 53 (7) is usually triangular or semi- 

 circular in shape, it is formed of a variable number of 

 segments, which differ from those of the thorax in being 

 fused together and immovable ; the segmentation being 

 shown by grooves. Just as in the thorax, the pygidium 

 possesses a central part or axis, and lateral portions ; the 

 axis may reach quite to the posterior extremity or only 

 part of the way. The margin of the pygidium may be 

 even or entire, or it may be provided with a posterior 

 spine or with lateral spines. 



The appendages of the trilobites were for a long time 

 unknown, since only a very few specimens showing the 

 ventral surface have been found ; the most satisfactory of 

 these is Asaphus megistos (fig. 54) from the Ordovician 

 Beds of Ohio. In this each segment of the thorax and 

 pygidium was found to bear a pair of jointed appendages. 

 Further information on this subject has been obtained by 

 Mr C. D. Walcott, by means of cutting thin sections of 

 rolled up specimens of Galymene and Cheirurus. In this 

 way he found that on the ventral surface of the body there 

 was a thin membrane, which was strengthened by trans- 

 verse arches to which the appendages of the thorax and 

 pygidium were attached ; these each consisted of six or 

 seven joints, and at the base a spiral gill was attached. 

 The head also carried four pairs of jointed appendages. 



The Trilobites are more closely related to the Xipho- 

 surans than to any other Arthropods, and since these are 

 now regarded by many zoologists as belonging to the 



