DEVELOPMENT OF SOME SILURIAN BRACIHOPODA 379 



Oeeasionally individuals of large growth show a greater 

 length than breadth, presenting an elongate form, but this 

 variation seems to be due to more rapid axial growth after 

 the attainment of adult size, and does not manifest itself in 

 the incompletely developed shells. 



Incipient Form (Plate XXI, figures 1, la, 2, "III). — The 

 minute shell which appears to have been the initial form for 

 the species has a circular outline and depressed convex 

 valves. The ventral valve is evenly convex, with the beak 

 erect, short, and broad. The cardinal area is low, the foram- 

 inal aperture triangular, reaching to, but not encroaching 

 upon the beak. The deltidial plates are absent. Dorsal 

 valve with the beak not incurved, but inconspicuous. Neither 

 valve bears any trace of a median elevation or depression. 



Developmental Varintions. 



General Form and Outline. — An inclination toward a lentic- 

 ular form and circular outline is noticeable in all immature 

 individuals. Until a size of about 18 x 18 mm. is attained, 

 there is rarely, if ever, any trace of the strong marginal fold 

 of maturity. 



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 

 incurved, 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 appear 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 thereafter above the dorsal valve is enclosed 



