ORIGIN AND SIGNIFICANCE OF SPINES 



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diversified into a row of spines, as represented in figure 6. 

 Further, the surface may be reticulate, with longitudinal 

 and transverse lines, and at the points of intersection, nodes 

 and often spines are formed after the manner shown in 

 figures 7-12. The longitudinal or vertical lines may become 

 obsolete, leaving the spines to be borne on the transverse or 



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Figures 7-12. — Diagrams ; showing growth and differeutiation of ornament 

 into spines. 7, surface with parallel lines ; 8, surface with regular reticulate lines ; 

 9, same, witli spines developed at the points of intersection ; 10, same, with the 

 yertical lines obsolete, but still represented by the vertical rows of spines; 11, 

 same, with the horizontal lines obsolete, but still represented by the horizontal 

 arrangement of the spines ; 12, same, with all lines obsolete, but both series 

 represented by the vertical and horizontal arrangement of the spines. 



horizontal lines (figure 10). In other cases the horizontal 

 lines disappear, leaving the spines on the vertical lines 

 (figure 11). Finally, both horizontal and vertical lines be- 

 come obsolete, and then only the spines remain, as shown in 

 figure 12. 



The indirect production of spines is not always evident, for 

 if the ontogeny or phylogeny of the individual is unknown, 



