SILURIAN BRACHIOPODA . 67 



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 foramen, 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 

 inwards, and often to a large degree concealed. 



Meristella rectirostra. Hall, 1882 



Plate VII, Figs. 4, 5, 11-13 

 , Hall. Eleventh Ann. Rept. 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 confounded with undeveloped indi- 

 viduals of Meristina 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. recti- 

 rostra 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 sufificiently complete to permit the 

 statement that, were younger forms accessible, they would probably add 

 little to our knowledge of the developmental changes. The series begins 



