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



the spiny characters of early adolescence are added to the 

 previous stages, and in later stages, the spines become more 

 and more emphatic. 



In Spondylus, the prodissoconch is the same simple form, 

 and is succeeded by a nearly smooth Pecten-like stage, during 

 which the animal was free (figure 29). After fixation, the 

 growth is very irregular and ostrseiform for a time, until the 

 shell rises above the object of support, when all the most char- 

 acteristic 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). 



29. 



31. 



Figure 29. — Young Spondylus princeps. Right valve, showing pecteniform 

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

 presented by R. T. Jackson, x 3. 



Figure 30. — Side view of Spondylus calcifer, about one- third grown, showing 

 the characteristic spinous growth. -J. 



Figure 31. — Side view of Spondylus calcifer, showing the greatly thickened 

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



It seems unnecessary to increase the number of examples 

 showing the ontogeny of spinose individuals. The Deer and 

 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 accomplished by' the removal of 

 the cause of antler growth and specialization, so that the 



