DEVELOPMENT OF BILOBITES 403 



maturity a gerontic tendency to obliterate the marginal sinus 

 is initiated; thus degenerating to an embryonal condition of 

 lobation similar to figure 14. 



The length of the hinge-line from an initial dimension 

 equal to the greatest width of the shell becomes equal to but 

 one-half the width of the shell in a specimen 3.5 mm. wide; 

 and in a full-grown individual, as represented by figure 25, 

 the hinge is not more than one-quarter the width of the shell. 

 From having sub-equal areas the change is rapid, so that in 

 a very early stage, but two or three removes from the initial 

 one of the series, the ventral area is the larger and the fissure 

 higher. This ratio progressively increases, and after the 

 shell reaches a length of 1.5 mm., the dorsal area ceases to 

 be a conspicuous feature. All areal growth and hinge exten- 

 sion end in the middle neanic period, and to this cause is due 

 the great disparity between the length of the hinge and the 

 width of the shell in ephebic individuals. The nepionic 

 shells show some extension of the cardinal angles, but the 

 auriculation does not become apparent until the lobation of 

 the valves is initiated. 



On account of the greater length of the incipient dorsal 

 valve and consequent obliquity of the area, the fissure and 

 area of that valve may be seen when the shell is viewed from 

 the ventral side, as in figure 10, and, consequently, the ven- 

 tral area is concealed from the dorsal aspect, as shown in 

 figures 3-9 and 11-15. This is a remarkable reversion of 

 characters, and one which appears to be of considerable 

 significance from a phylogenetic standpoint. 



The radiating striae first appear on the lower half of the 

 initial shell of the series, indicating that in an earlier condi- 

 tion the shell was smooth. The striae appear in pairs. The 

 first two striae extend to the antero-lateral borders. An 

 additional intercalated pair is next introduced, together with 

 a single one on each side between the primary radii and the 

 cardinal border. The number after this stage is more rapidly 

 increased by increment in the cardinal lateral areas than in 

 the median region. 



