DEVELOPMEXT OF SOME SILURIAN BRACHIOPODA 337 



aperture for the protrusion of the pedicle; by the peculiar 

 development of the articulating processes of both valves, the 

 entire cardinal margin is closed, and therefore the passage 

 between the internal plate and the surface of the valve may 

 have been for the use of this organ; or, it may be suggested 

 that as this space is rather too narrow and explanate for such 

 a purpose, Didyonella may have been attached by the sub- 

 stance of the shell, the internal shelf acting as a support to 

 the strain upon the umbo and as a protection to the animal 

 in case the shell were broken from its attachment. 



Anastrophia internascens Hall, 1879. 



(Plate XVII, figures 14-16.) 



Hall. Twenty-eighth Ann. Kept. N. Y. State Mus. Nat. Hist., 



p. 168, pi. 26, figs. 41-49, 1879. 

 Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 311, pi. 26, 



figs. 41-49, 1882. 



In tracing the development of this species the principal 

 feature to be noticed is that the elemental shell conforms with 

 the type of an ordinary brachiopod, such as Camarotoechia ; 

 that is, the dorsal valve, although somewhat the more con- 

 vex, is smaller than the opposite valve, while in the mature 

 state the dorsal valve is considerably larger and projects 

 beyond the beak of the ventral valve. It is the development 

 of this character which constitutes the most conspicuous 

 change in the shell in its growth from the young to the fully 

 mature condition. 



Specific Characters. 



Mature Torm (Plate XVII, figures 15, 16, 16 a). — Shell 

 ventricose. Outline transversely sub-elliptical, sometimes 

 nearly as long as wide. 



Ventral valve convex, depressed in front, forming a more 

 or less defined sinus which carries four or five of the plica- 

 tions; beak short, acute; area short, broadly triangular, 

 usually not exposed. 



22 



