SILURIAN BRACHIOPODA . 75 



Beak. The low but erect ventral beak of the initial shell, has, in the 

 next stage of growth, become inflected and obtuse, not, however, so as to 

 conceal the foramen, which remains apparent above the apex of the dorsal 

 valve, until the rapid increase in convexity, which immediately precedes 

 maturity, sets in. Thereafter, the ventral beak becomes more closely in- 

 curved, and thrust over upon the dorsal valve, to the loss of all external 

 trace of the cardinal area. 



Foramen. The elemental hiatus is shown in the initial shell and the 



subsequently developed deltidial plates in the next growth-stage. In the 

 latter case, the foramen has become nearly if not quite enclosed and has 

 also encroached upon the apical portion of the valve, which forms about 

 one-half its periphery. In all subsequent stages of growth, the deltidial 

 plates are concealed, and whatever portion of the foramen appears there- 

 after above the dorsal valve is enclosed by the circumbonal tract. With 

 the approach of maturity, this gradually disappears, and at full growth, 

 every trace of it has become obliterated. 



Spirifer crispus, Hisinger, 1826 



Plate VI, Figs. 6, 7 



Spirifer crispu.s, van simplex. Hall, 1879 



Plate VI, Figs. 4, 5 



Spirifer bicostatus, Vanuxem, 1842, van petilus. Hall, 1879 



Plate VI, Figs. 1-3 



Spirifera crispa. Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 157, pi. 24, 



figs. 6-12, 19. 1879. 



, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 295, pi. 24, figs. 6-j2, 19. 1882. 



Spirifera crispa, var. simplex. Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 



157, pi. 24, figs. 1-5. 1879. 



, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 286, pi. 24, figs. 1-5. 1882. 



Spirifera bicostata? \2iX. petila. Hall. Trans. Alb. Inst., vol. x, Abstract, p. 15. 1879. 



, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 297, pi. 27, figs. 8, g. 1882. 



The three forms which are here treated together, are closely allied in 

 all their general characters. It is in their initial stages, however, that the 



