C. E. Beecher — Origin and Significance of Spines. 9 



Elk, etc., the spines are moulted, or shed, periodically. In 

 their various stages, these types (1 and 2) can be studied only 

 by means of separate specimens consecutive in age, or by 

 observing the metamorphoses in one individual. (3) In the 

 shells of Brachiopods and Mollusca, the stages of growth of 

 the individual are generally retained throughout life, and the 



13. 14. 



Figure 13.— Summer shoot of Barberry, showing the gradations between leaves 

 and spines. The arrow indicates the direction of growth. (After Gray.) 



Figure 14. — Profile of one of the primary rays of Spondylm imperialis, show- 

 ing the series of spines. The arrow indicates the direction of. growth. 



Figure 15. — Example of spine growth by simple increscence. Horn (left) and 

 horn-core (right) of Ox. (After Owen.) 



Figure 16. — Stages of spine growth by successive replacement. Antler series 

 of Red Deer, at ages of 1,2, 3. etc., years. (After Owen.) 



Figure 17. — Stages of spine growth by serial repetition. Profile of a series of 

 spines on one of the primary radii of Spondylus imperialis. 



Figure 18. — Stages of spine growth by decrescence. Transformation of leaves 

 into spines in Beroeris vulgaris. (After Gray.) 



