XX ILLUSTRATIONS 



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 . ... 1.5 



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 oi Avicida 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. Aitheya 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. Cyatkophycus reticulalus. Ordovician 49 



38. Dictyospongia Conradi. Devonian 49 



39. Hydroceras tuberosum. Devonian . 49 



