382 STUDIES IN EVOLUTION 



men, as one abnormal example has three plications on one 

 side of the median fold and four on the other. 



Spirifer radiatus Sowerby, 1825. 



(Plate XX, figures 9-11.) 



Spirifera radiata Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. 



Hist., p. 157, pi. 24, figs. 20-30, 1879. 

 Hall. Eleventh Ann, Rept. State Geol. Indiana, p. 296, pi. 24, 



figs. 20-30, 1882. 



The series of specimens representing the gradation in size 

 from very young to mature forms is quite complete, but, 

 unfortunately, the characters of the most interesting feature, 

 the deltidium, are not well shown. The foramen is usually 

 but partially closed when the shell reaches nearly its full 

 dimensions, and at this period the beak of the ventral valve 

 is so incurved and thickened that the detailed development 

 of the deltidial plates is obscured, and rendered difficult of 

 interpretation. 



This species has been so fully discussed in all its aspects, 

 on account of its wide geographical distribution and varied 

 physical conditions, that a diagnosis of the adult form is un- 

 necessary in this place (vide Plate XX, figures 11, 11 a). 



Incipient Form (Plate XX, figures 10, 10 a). — The small- 

 est example yet detected has a length of 1.5 mm. The 

 specimen is not well preserved, and the one used for illustra- 

 tion and description is somewhat larger, measuring 2 mm. in 

 length. The differences appear to be so slight that the 

 characters of the larger may well be applied to the smaller 

 individual. 



The shell is nearly circular and flattened, with the beaks 

 not incurved but directed outward. The area of the ventral 

 valve is broad, triangular, open, and extends nearly the 

 entire length of the cardinal line. The incipient dorsal fold 

 and ventral sinus extend nearly to the beaks, and on each 

 side there are about ten radiating striae. Radii are also 

 present on the fold and in the sinus. 



