172 



THE CHICAGO ACADEMY OF SCIENCES. 



tinuous around the anterior extremity of the glabella, the area 

 in front of the hypostome is partly occupied by a separate 

 plate which is sometimes visible dorsally. The remaining 

 characters of the 'ventral side of the head, so far as they are 

 known, will be described later in connection with the discussion 

 of the ventral appendages. 



The thorax of the trilobites consists of a series of short, 

 transversely elongate, articulating segments, which vary in 

 number among different genera from two to twenty -nine. Each 

 thoracic segment consists of the central or axial portion lying 

 between the dorsal or axial furrows, and the lateral or pleural 

 portions. The axial portion is usually more or less strongly 

 convex transversely ; the pleura are usually depressed below the 

 axis and have a proximal or inner more or less flattened portion, 

 and a distal or outer portion which is bent more or less abruptly 

 downward and backward, terminating in a backwardly pointing 

 angular extremity. The articulation between the thoracic 

 segments is in the axial region where each segment bears an an- 

 terior extension somewhat beneath the general surface and 

 separated by a furrow, which passes beneath the incurved pos- 

 terior margin of the segment in front. The pleura of the thoracic 

 segments of many trilobites are grooved diagonally, the grooves 

 originating near the anterior margin at the dorsal furrow and 



from there passing obliquely 

 outward and backward, often 

 extending to the angular dis- 

 tal extremities. In some gen- 

 era, however, the pleura are 

 smooth throughout. 



The posterior abdominal 

 region of the trilobite is com- 

 monly known as the pygidium. 

 Like the cephalon and thor- 

 ax, it has^a central axial re- 

 gion or lobe, with two lateral 

 or pleural lobes separated 

 from the axis by the axial 

 or dorsal furrows; it is usu- 

 ally more or less arched'trans- 

 FIGURE 2. Thorax of a trilobite: versely, especially in the axial 

 a, axial region; p pleural regions; af, reg ion, though in some forms 

 axial or dorsal furrows; s, segments; 

 pff t pleural grooves. the pleura are more or les 



