THE NATURAL HISTORY SURVEY. 



i.73 



'completely flattened. In some genera the true elevated portion 

 of the axis is short, being produced posteriorly into a post-axial 

 region which is usually limited by the continuation of the dorsal 

 or axial furrows, but which is not elevated above the surface 

 of the pleural lobes. The pygidium consists of a single piece 

 which has originated through the coalescence of several originally 

 articulating segments. The original segmentation of the pygi- 

 dium is usually more or less distinctly preserved, though in 

 *some forms it has become completely ob- 

 solete. It is somewhat difficult to detect 

 the line of union between the thorax and 

 pygidium in those forms with a strongly 

 'segmented pygidium, except in disarticu- 

 lated specimens ; but in those forms with 

 obsolete segmentation of the pygidium, 

 "this line stands out strongly. The seg- 

 ments of the pygidium, like those of the 

 thorax, may be either grooved longitudin- 

 ally, or may be plain. There is usually a 

 more or less definite relationship between 

 the size of the pygidium and the number 

 of thoracic segments, those trilobites with 

 .a large number of thoracic segments hav- 

 ing a small pygidium, and conversely, 



FIGURE 3. Pygid- 

 ium of a trilobite: a, 

 axis; p, pleural lobes; 

 s, segments of axis and 

 pleura; pa, post-axial 

 region; af, axial or 

 dorsal furrows; c t cau- 

 dal spine; m, marginal 

 border. 



forms with a smaller number of thoracic segments having a 

 relatively larger pygidium. 



The lateral and posterior margins of the trilobite pygidium 

 are frequently entire, the outline forming a regular sub-elliptical, 

 sub-circular or parabolic curve, or it may be sub-triangular in 

 outline ; in other forms the distal extremity of each pleural seg- 

 ment is produced into a spine-like process of greater or less 

 length, and frequently a caudal spine, sometimes of great length, 

 extends posteriorly in line with the axis from the posterior 

 margin. The length of the elevated portion of the axis of the 

 pygidium varies greatly, in some forms it being less than one- 

 fourth the pygidium while in others it reaches to the posterior 

 margin and may even be produced backward in the caudal 

 spine. The segmentation of the pleura may reach to the lateral 

 margins or it may stop short of the margin and leave a more or 

 less flattened marginal border. The caudal spine, when present, 

 may be a mere posterior extension of the marginal border or it 

 may be a posterior extension of the axis proper. 



