384 STUDIES IN EVOLUTION 



by accretion along the inner edges of the deltidial plates. 

 The initial state is represented by a narrow elongate lamina 

 on each side of the triangular area. Further growth pro- 

 duces a triangular plate, and to the form of the triangle is 

 due the apparent growth of the pseudo-deltidium from the 

 apex of the fissure downward, or from the base of the fissure 

 toward the beak of the ventral valve. 



The accompanying diagrammatic outlines (figure 129) serve 

 to illustrate the changes and the final results. 



129 



A A 



FIGURE 129. Deltidial development in Spirifer. 



Figure 1 represents an area in an early stage of growth, 

 with a narrow deltidial plate on each side, alike for each 

 series. 



Figure 2 shows scalene triangular plates, with the shortest 

 side at the base of the area. 



Figure 2' shows plates with the two free edges more nearly 

 equal. 



Figure 2" presents narrow triangular plates as in figure 2, 

 but with the shortest edges in the apex of the area. 



In figures 3, 3', 3", the growth has continued in the direc- 

 tion initiated in the preceding stage, and the apex of the 

 area has been partially filled from the internal thickening of 

 the beak. 



