346 STUDIES IN EVOLUTION 



and, consequently, the sub-ovate 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 3.5 x 3 mm. and 6 X 5.5 mm., the ventral 

 valve still retains a slightly greater convexity, but the ante- 

 rior margin is entire. 



Beak and Foramen. — The erect and straight beak of the 

 incipient shell becomes slightly incurved toward maturity, 

 but the cardinal area remains high, exposing the triangular 

 foramen at all stages of growth. Deltidial plates make their 

 appearance early, but never develop sufficiently to meet and 

 enclose the pedicle-aperture, a feature indicative of arrested 

 development, 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 indi- 

 cates that the initial shell may have been smooth, as it 

 has been shown to be in R. indianensis. The subsequent 

 addition of plications takes place slowly and from the cardinal 

 margins. 



Camarotoeohia indianensis Hall, 1863. 

 (Plate XVII, figures 1 7-28.) 



RhyncTionella indianensis Hall. Twenty-eighth Ann. Rept. N. Y. State 

 Mus. Nat. Hist., p. 163, pi. 26, figs. 12-22, 1879. 



Hall. Eleventh Ann. Rept. State Geol. Indiana, p, 306, pi. 26, 



figs. 12-22; pi. 27, figs. 4-6, 1882. 



Camarotoeohia indianensis is, beyond a doubt, the most 

 prolific species in the rich fauna of the Waldron beds, and 

 by virtue of this fact it has been possible to ascertain the 

 developmental phases through illustrative series of excep- 

 tional completeness. It is noteworthy that the mature shell 

 of this Camarotoeohia presents variations from the adult type, 

 which are so great that in a certain sense they might be 



