372 STUDIES IN EVOLUTION 



been displaced and lost. Careful search among the smallest 

 individuals has shown no trace of the inceptive triangular 

 outline of the pedicle-groove existing in other species before 

 the formation of the deltidial plates. It is an important fact 

 that the foramen begins to assume its mature condition so 

 early in the history of the shell, although its development 

 was evidently in conformity with the general type. 



The subsequent development of the deltidial plates changes 

 the form of the foramen to that of a circle, as shown in figures 

 22 and 23. In the early life of the shell the plane of the 

 foramen is in, or parallel to, the axial plane; at maturity, 

 before any obesity or senile thickening takes place, the fora- 

 men, in becoming less elongate, truncates the apex of the 

 valve, and makes a large angle (sometimes almost 90) with 

 the axial plane; subsequently, with increase in obesity, it 

 becomes again more nearly parallel to this plane. In the 

 last condition the deltidial plates are curved inward, and 

 often to a large degree concealed. 



Meristina rectirostris Hall, 1882. 



(PLATE XXI, figures 4, 5, 11-13.) 



Meristella rectirostra Hall. Eleventh Ann. Kept. State Geol. Indiana, 

 p. 301, pi. 27, figs. 10-14, 1882. 



This small species is one of the less abundant of the 

 brachiopods of this fauna, and probably has often been con- 

 founded with undeveloped individuals of Whitfieldella nitida. 

 It presents, however, adult features which will not allow it 

 to be confounded with that species, and although some diffi- 

 culty arises in separating the diminutive forms of the two 

 species, M. rectirostris is characterized by the absence of 

 deltidial plates in every stage of its existence. 



The series representing this species does not include stages 

 of growth as early as in some of the others, but is sufficiently 

 complete to permit the statement that, were younger forms 

 accessible, they would probably add little to a knowledge 

 of the developmental changes. The series begins with indi- 



