xx ILL US TRA TIONS 



FIGURES IN TEXT 



FIGURE PAGE 



1-5. Different stages of growth of a spine 10 



6. A profile of a single radiating ridge of Spondylus princeps ; 



showing the series of flattened spines 10 



7-12. Diagrams ; showing growth and differentiation of ornament 



into spines . 11 



13. Summer shoot of Barberry ; showing the gradations between 



leaves and spines 12 



14. Profile of one of the primary rays of Spondylus imperialis ; show- 



ing the series of spines 12 



15. Example of spine growth of simple increscence 13 



16. Stages of spine growth by successive replacement .... . 13 



17. Stages of spine growth by serial repetition 13 



18. Stages of spine growth by decrescence 13 



19. Sector; showing in diagram the multiplication of radiating lines 



by interpolation 15 



20. Profiles of spines produced on the various radii at the four 



zones; as indicated in the preceding figure 15 



21. Simple spine 16 



22. Spine, with lateral spinules 16 



23. Spine, with forked apex and lateral spinulose spinules ... 16 



24. Prodissoconch of Ostrea virginiana . 20 



25. Each stage of Avicula sterna 20 



26. Young of Avicula sterna ; showing the beginning of spine growth 20 



27. Young Saxicava arctica 20 



28. Young Anomia aculeata 20 



29. Young Spondylus princeps 20 



30. Side view of Spondylus calcifer ; about one-third grown ; show- 



ing the characteristic spinous growth 21 



31. Side view of Spondylus calcifer ; showing the greatly thickened 



right valve and the entire absence of spines over the whole 



shell 21 



32. Attlieya decora, a diatom, with spines from the angles .... 44 



33. Difflugia acuminata, a freshwater rhizopod; showing spiniform 



projection of the fundus 44 



34. Difflugia constricta, a freshwater rhizopod, with rounded fundus 44 



35. The same; showing a single spine on the fundus 44 



36. The same; showing two spines 44 



37. Cyaihophycus reticulatus. Ordovician 49 



38. Dictyospongia Conradi. Devonian 49 



39. Hydroceras tuberosum. Devonian 49 



