ORIGIN AND SIGNIFICANCE OF SPINES 



79 



Likewise, if the pleura or their distal ends are directed 

 posteriorly nearly parallel to the axis, the mechanical neces- 

 sities of motion require that the portions of the free segments 

 pointing backward should be free, thus making the ends of 



61 



62 



63 



64 



65 



Figure 61. — Ilkcnua (Octillo'nus) Hishgeri, Ordovician, Bohemia ; a trilobite ; 

 showing spiniform pleural extremities of first thoracic segment, correspouding 

 to the genal spines of the cephalon. X f. (After Barrande.-*) 



Figure 62. — Chelrurus infsigyiis, Silurian, Bohemia ; pygidium and six 

 thoracic segments. X f • (After Barrande.) 



Figure 63. — Deipkon Forbesi, Silurian, Bohemia; entire specimen; show- 

 ing spiniform pleura of segments corresponding in direction to those of the 

 pygidium. (After Barrande.) 



Figure 64. — Lirhas smhra, Silurian, Bohemia ; pygidium, with three 

 thoracic segments ; showing spiniform ends of pleura. X f. (After Barrande.) 



Figure 65. — Paradoxides spinosus, Cambriau, Bohemia; pygidium and six 

 free segments. X f- (After Barrande.) 



the thoracic pleura generally appear as retrally curved spini- 

 form extensions. Extreme examples of retrally directed 

 pleura accompanied by small pygidia are shown in Para- 

 doxides (figure 65), Jlolmia, Olenellus, Elliptocephala^ etc. 



