DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 341 



details, but they may be described as follows: Attenuate, 

 sub-spatulate. Ventral valve with erect, straight beak , car- 

 dinal area high, convex, with a prominent dorsum. Dorsal 

 valve flattened or slightly sinuate. Features of the cardinal 

 area not discernible; from analogy, the foramen would be 

 triangular and unobstructed. In figures 9-9 6, which show a 

 secondary stage of growth in the shell, the portion included 

 within the first growth-line will represent very well the char- 

 acters of the primitive shell. 



General Developmental Characters. 



The gradual incurvature of the beak and consequent con- 

 cealment of the ventral foramen may be assumed from the 

 foregoing. It harmonizes with the associated species of the 

 same genus in the slight variation in the form and propor- 

 tions of the foramen in consecutive stages of growth, as well 

 as in the reversal of the embryonic fold and sinus to the 

 mature sinus and fold. The plications of the latera seem to 

 appear simultaneously after the first varix, as shown in the 

 figures referred to, and their number does not change mate- 

 rially until maturity. The embryonic sulcus on the dorsal 

 valve, correlate with the ventral dorsum in the primitive 

 stage, is continued at maturity into the median sulcus sepa- 

 rating the two plications of the dorsal fold. 



Camarotcechia neglecta Hall, 1852. 

 (PLATE XVIII, figures 3, 6-8.) 



Rhynchonella neglecta Hall. Twenty-eighth Ann. Kept. N. Y. State Mus. 



Nat. Hist, p. 162, pi. 26, figs. 1-6, 1879. 

 Hall. Eleventh Ann. Kept. State Geol. Indiana, p. 305, pi. 26, 



figs. 1-6 ; pi. 27, fig. 3, 1882. 



For a species so abundant as this in the Waldron fauna, 

 the diagnostic features are retained with unusual persistence 

 within very narrow limitations. Unlike its associate, 0. in- 

 dianensis, which it almost equals in numerical representa- 

 tion, there are no well-established and perduring variations 



