SILURIAN BRACHIOPODA 73 



WHITFIELDIA MARIA, Hall, 1863 



PLATE VII, FIGS. 1-3 



Meristella Maria, Hall. Trans. Albany Inst. vol. iv, p. 212. 1863. 



Meristina Maria, Hall. Twenty-eighth Ann. Kept. N. Y. State Mus. Nat. Hist., p. 159, pi. 25, 



figs. 8-12. 1879. 



, Hall. Eleventh Ann. Kept. State Geol. Indiana, p. 299, pi. 25, figs. 8-12. 1882. 

 *Whitfieldia tumida (Dalman Sp.), Davidson. Supp. Brit. Sil. Brach., p. 107. 1882. 



This species may be regarded as presenting a general external form and 

 effect diametrically opposed to that in Meristella rectirostra. To the erect, 

 attenuate, acute beak, open pedicle-aperture, shallow valves, and asinuate 

 anterior margin of the latter, the full, rounded, incurved beak, concealed 

 cardinal area, ventricose valves, and strongly sinuate anterior margin of 

 W. niaria are strongly contrasted. Between the mature characters of these 

 two species, Meristina nitida is conspicuously mediate. 



Immature specimens of W. maria are far from abundant. Indeed, 

 our series shows only about thirteen different grades of development, and 

 the smallest individual which can be referred with certainty to the species 

 measures 6x6 mm. (adult 29 mm. in length by 32 mm. in width). 



We have, however, assigned to the species a minute embryo measuring 

 75 x -75 mm., and if this is correctly done, the embryos of this species in the 

 earliest stages of growth differ from- those of the other non-plicate species 

 here discussed in a much stronger tendency toward a circular outline. 



The beak of the ventral valve becomes incurved, and the cardinal area 

 obscured very early, so that the discussion of the development of these 

 parts is necessarily much curtailed. 



*The late Mr. Davidson identified the Waldron species with the Atrypa tumida of Dalman, 

 the type of his genus Whitfieldia. We use the term Wh.itfi.Mia here for convenience, but if the 

 identity of the American and European forms is established, Atrypa tumida will have been taken 

 as the type of two {Meristina, Hall, Whitfieldia, Davidson), if not of three {Meristella, Hall) 

 genera. 



10 



