DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 321 



always leaving room for the protrusion of the pedicle. In 

 immature conditions the cardinal process is attached to the 

 shell only at the apex of the foramen, but with maturity it 

 comes in contact with the sides of the foramen, and at this 

 stage entirely fills the dorsal aperture. With the increasing 

 incurvature of the ventral beak and cardinal area, the aper- 

 tures of the two valves change their mutual angle, constantly 

 lessening it as growth advances. 



Plications. As noticed above, the earliest stages of growth 

 observed show the strise to be already developed on the shell, 

 five on the ventral and six on the dorsal valve. These plica- 

 tions are rapidly multiplied by interstitial addition, and at 

 maturity number from one hundred to one hundred and 

 thirty on each valve. 



Rhipidomella hybrida Sowerby, 1839. 



(PLATE XV, figures 13-18.) 



Orthis hybrida Hall. Twenty-eighth Ann. Kept. N. Y. State Mus. Nat. 



Hist., p. 149, pi. 21, figs. 18-25, 1879. 

 Hall. Eleventh Ann. Kept. State Geol. Indiana, p. 285, pi. 



21, figs. 18-25, 1882. 



Rhipidomella hybrida passes through primary develop- 

 mental stages which are essentially identical with those 

 already described for Dalmanella elegantula. Sufficient has 

 been said in that connection in regard to the similarity and 

 probable identity of the earlier embryonic stages of the shell 

 of both species, the origin of the entire specific difference 

 which is so apparent in the later and mature periods of devel- 

 opment lying in the unequal growth of the valves in con- 

 vexity. This increase is relatively greater in the dorsal valve 

 of R. hybrida than in that of D. elegantula, and less in the 

 ventral valve of the former than in that of the latter species. 

 Thus R. hybrida is a more discoid, lenticular shell, showing 

 but slignt evidence of a median fold and sinus and carrying 



21 



