ORIGIN AND SIGNIFICANCE OF SPINES 



21 



characteristic features of surface ornamentation become fully 

 developed (figure 30). As the shells approach maximum 

 growth, the spines gradually become shorter, and in old age 

 none are developed, even those of early growth being removed 

 by the action of boring animals and by solution (figure 31). 



It seems unnecessary to increase the number of examples 

 showing the ontogeny of spinose individuals. The Deer and 



29 



31 



Figure 29. — Yonng Spond;/lus princeps. Right valve ; showing pecteniform 

 stage succeeded by ostriuiform growth. Taken from apex of adult specimen ; 

 presented by R. T. Jackson. X 3. 



Figure 30. — Side view of S]>ondylus calcifer, about one-third grown; show- 

 ing the characteristic spinous growth. \. 



FiGCKE 31. — Side view of Spondi/hts calcifer ; showing the greatly thickened 

 right valve and the entire absence of spines over the whole shell. \. 



the Ox may be again cited in this connection. Both are 

 born without horns, but during adolescence the antlers of 

 the Deer become longer and more complicated with each 

 renewal, while the horns of the Ox are longer and more 

 twisted. In old age, when the Deer has passed his prime, 

 the antlers are more obtuse, and exhibit a tendency toward 

 decline and obliteration. Suppression of the antlers is accom- 

 plished by the removal of the cause of antler growth and 

 specialization, so that the unsexing of the male results in 



