SILURIAN BRACHIOPODA 41 



the ventral being the more convex. Ventral valve with an erect, straight 

 beak ; apex acute, cardinal margins sloping rapidly forward, and slightly 

 excavate. Foramen simple, triangular, free from deltidial plates, encroach- 

 ing at its apex slightly upon the umbo ; foraminal margins somewhat thick- 

 ened. Dorsal beak erect but inconspicuous, full and rounded. Dorsal 

 valve depressed anteriorly along the median line, this depression correspond- 

 ing with the broad and low dorsum of the opposite valve. Surface of each 

 valve marked by eight single, rounded plications, which extend two-thirds 

 the distance from the anterior margin of the beak, leaving the circumbonal 

 area smooth. 



DEVELOPMENTAL VARIATIONS 



General Form and Outline. As growth advances, the development 



is more rapid transversely than longitudinally, and, consequently, the subo- 

 vate incipient shell becomes, at maturity, broadly transverse. The prominent 

 dorsum of the ventral valve in the embryo, is manifest at maturity only in 

 the rounded and prominent beak, and the embryonal sinus in the dorsal 

 valve becomes so thoroughly obsolete at maturity as to be unnoticeable. 

 In stages of development between the dimensions 3.5x3 mm. and 6x5.5 mm., 

 the ventral valve still retains a slightly greater convexity, but the anterior 

 margin is entire. 



Beak and Foramen. The erect "and straight beak of- the incipient 

 shell becomes slightly incurved toward maturity, but the cardinal area re- 

 mains high, exposing the triangular foramen at all stages of growth. Del- 

 tidial plates make their appearance early, but never develop sufficiently to 

 meet and inclose the pedicle-aperture, a feature indicative of arrested de- 

 velopment, and equally true of the other members of the genus here 

 discussed. 



Plications. The fact that the eight plications' on each valve of the 

 incipient shell do not reach the umbones, indicates that the initial shell may 



have been smooth, as we have shown it to be in R. indiancnsis. The 

 6 



