SILURIAN BRACHIOPODA ^i 



individuals, but they do not serve to close the fissure which remains 

 open in all stages of growth. 



S. crispus, van simplex reaches a width of 8 mm., and ^. crispus often 

 measures 22 mm. in width. Occasionally, a specimen of 6*. crispus of the 

 usual size is found with but seven plications on the dorsal valve, suggesting 

 a very large example of the variety, or that the characters of the smaller 

 and simple form are sometimes continued far beyond the period when they 

 usually disappear. Also, the features both of the species and variety may 

 be combined in a single specimen, as one abnormal example has three pli- 

 cations on one side of the median fold and four on the other. 



Spirifer radiatus, Sowerby, 1825 



Plate VI, Figs. 9-1 i 



Spirt/era 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 unnecessary in this place (vide plate 6, figs. 

 1 1, I la). 



Incipient Form (plate 6, figs. 10, 10^). The smallest example yet 



detected has a length of 1.5 mm. The specimen is not well preserved, and 

 the one used for illustration and description is somewhat larger, measuring 



