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. lUoenus (Octillanus) Hisingeri, Ordovician, Bohemia ; a trilobite ^ 

 showing spiniform pleural extremities of first thoracic segment, corresponding 

 to the genal spines of the cephalon. X f- (After Barrande. 4 ) /?(*, 0^ /^ 



FIGURE 62. Cheirurus msignis, Silurian, Bohemia ; pygidium and six. 

 thoracic segments. X f. (After Barrande.) 



FIGURE 63. Deiphon Forbesi, Silurian, Bohemia; entire specimen; show- f^fT^ 

 ing spiniform pleura 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. Paradoxides spinosus, Cambrian, Bohemia.; pygidium and six ^ j Jfr 



free segments. X/f- {After Barrande.) ^/>^ 



ju&JC*^ji%^ ^^Ski^^J ^^^^aJ^ff^ CS--#*&* 



ie tnoracic 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, Elliptocephala, etc. 



