402 STUDIES IN EVOLUTION 



After considerable labor and search, about a thousand indi- 

 viduals have been obtained. From this number it has been 

 possible to select a series of over forty specimens, showing 

 stages of growth ranging from shells a little less than one- 

 half a millimetre in length to a length of nine millimetres, 

 thus representing the development between these limits by 

 almost insensible gradations. 



Developmental Changes in Bilohites various. 



In the youngest specimens yet detected, measuring .49 

 mm. in length, and semi-elliptical in outline, the dorsal 

 valve is longer than the ventral; the hinge is equal to the 

 greatest width of the shell ; both areas are high, sub-equal, 

 and perforated by a triangular fissure in each valve. In rare 

 instances the pedicle covering or pseudo-deltidium is appar- 

 ently retained in young shells. 

 Figure 131 of the ventral area 

 shows the fissure and pedicle cov- 

 ering, with the foramen at the 

 apex of the beak. The covering 

 is soon absorbed or abraded dur- 



FiGLRE \3l.— Bilobites van- . , ^, , ^, 



cus, Conrad; ventral area. X 25. "^g subsequent growth, and the 



pedicle then emerged through the 

 fissure below. None of these characters obtain in tlie neanic 

 or ephebic stages, which are represented by a cordate, bilobed 

 shell; dorsal valve shorter than the ventral; hinge-line much 

 shorter than the width of the shell, and an inconspicuous 

 dorsal area without a fissure. 



The series of outlines (Plate XXIII, figures 11 to 26) 

 drawn to the same scale, illustrate both the important 

 changes which take place in the general form, and the corre- 

 sponding increase in size from stage to stage. The rounded 

 frontal margin of figures 11 and 12 becomes straight in figure 

 13, and in figure 14 a gentle sinus is apparent, which is pro- 

 nounced in figure 15, and thereafter is the conspicuous char- 

 acter of the entire shell up to the ephebic stage represented 

 by figure 23. Figures 24 and 25 show that upon reaching 



