10 C. E. Beecher — rig in and Significance of Spines. 



successive development of spines may be studied, therefore, 

 in a single example. (4) Spines produced by suppression, as 

 in the Barberry, express their origin through a series of gra- 

 dations between separate parts ; while in others, suppression is 

 brought about by the loss of structures. 



The first type mentioned develops horns or spines by simple 

 increscence (figure 15) ; for example, the Ox : the second, by 

 successive replacement (figure 16); as in the Deer: the third, 

 by serial repetition (figure 17); for example, Spondylus : the 

 fourth, by decrescence (figure 18) ; for example, the Barberry. 



Localized stages of growth. — By the multiplication of surface 

 ornaments through the process of interpolation, many Mollusca 

 present stages of spine development in two directions. (1) The 

 normal series is represented by the succession of spines along a 



19. 



20. 



J I 3 l i i i I 31 5 



_ Ii.iilt.fi. 



511151 1 in 1151 1 mn a 



/^Uli 41 ^ 



Figure 19. — Sector showing in diagram the multiplication of radiating lines by 

 interpolation. The two primary radii (1, 1) are the only ones continuing through 

 the whole four zones. The first zone has 2 radii; the second, 5; the third. 11 ; 

 and the fourth, 23. 



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

 zones, as indicated in the preceding figure. A, the spines on the two primary 

 iadii of the first zone; B, the spines on the second zone, showing the growth of 

 those on the two primary radii (1, 1), and the small spines on the newly interpo- 

 lated radii (2, 2, etc.); C, the spines on the radii in the third zone ; D, the spines 

 at the bottom of the fourth zone. The two large compound spines are on the 

 two primary radii. Their development may be traced by following them through 

 A, B, C, to D. The next three longest spines (2, 2, 2) are tricuspid, and represent 

 the stage of spine development attained by the spines on the radii which were 

 interpolated on the second zone. The next six smaller spines (3, 3, 3, etc.) are on 

 radii which were introduced on the third zone. The twelve small spines (4, 4, 4, 

 etc.) are on the radii introduced on the fourth zone. Thus there are four stages 

 of spine growth shown on the lower margin of the fourth zone, and these corre- 

 spond to the four stages exhibited by the series of spines on one of the primary 

 radii running through the four zones. 



