ORIGIN AND SIGNIFICANCE OF SPINES 79 



Likewise, if the pleura or their distal ends are directed 

 pnstcriorly neiirly parallel to the axis, the mechanical neces- 

 sities of motion riM^uire that the portions of the free segments 

 pointing backward should be free, thus making the ends of 



61 



62 



63 



64 



65 



FioiTiE 61 . — Illfpnus ( Octillftnus) Hisingeri, Ordovician, Bohemia ; a trilobite ; 

 showinr; spiiiiform pleural extremities of first thoracic segment, corresponding 

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



FiGHKE 62. — Chr-iriiru.i insignis, Silurian, Bohemia; jjygidium and six 

 thoracic segments. X J- (After Barrande.) 



Figure 6.3. — Deiphon Forbes!, Silurian, Bohemia; entire specimen; show- 

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

 pvgidlum. (After Barrande.) 



Figure 64. — IJrhas scahra, Silurian, Bohemia ; pygidium, with three 

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



Figure 6.5. — Pamcloxiilns spinosus, Cambrian, Bohemia; pygidium and six 

 free segments. X J- (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), Holmia, Olenellus, Mliptocephala, etc. 



