338 C. E. Beecher — Origin and Significance of Spines. 



the cephalon and pygidium, and its form is mechanically in 

 agreement with the requirements of the animal for bending 

 the body, and with the adjacent margins of the cephalon and 

 pygidium. In a way, it may be said that the segment is 

 moulded by the adjacent parts, and may therefore take its 

 form from the cephalon (figure 61), or from the pygidium, as 

 in the examples following : 



01 



62 



63 



65 



Figure 61. Illcenus (Octillcenus) Hisingeri, Ordovician, Bohemia. A Trilobite 

 showing spiniform pleural extremities of first thoracic segment, corresponding to 

 the genal spines of the cephalon. x £. (After Barrande 4 .) 



Figure 62. Cheirurus insignis, Silurian, Bohemia. Pygidium and six thoracic 

 segments, x f . (After Barrande.) 



Figure 63. Deiphon Forbesi, Silurian, Bohemia. Entire specimen showing 

 spiniform pleural of segments corresponding in direction to those of the pygidium. 

 (After Barrande.) 



Figure 64. Lichas scabra, Silurian, Bohemia. Pygidium with three thoracic 

 segments, showing spiniform ends of pleura, x f. (After Barrande.) 



Figure 65. Paradoxicles spinosus, Cambrian, Bohemia. Pygidium and six 

 free segments, x f . (After Barrande.) 



During growth, the new segments are added in front of the 

 anal segment, so that after the number of abdominal segments 

 is complete the thorax is increased by the successive addition 

 of what in earlier moults were pygidial segments. By this 

 means, the pygidium generally controls or determines the char- 

 acter of the segments of the thorax. If the pleura of the 

 pygidium are extended into spiniform processes, the pleural 

 ends of the segments are also spiniform, as in Lichas (figure 

 64), Ceraumcs, Cheirurus (figure 62), Deiphon (figure 63), 

 Acidaspis, Dindymene, etc. 



