BRACHIOPODA. 131 



arrangement in which the middle portion represents the path of the aperture. 

 This character is not only common to palseozoic "Discinas" generally, but is seen 

 in ScHizoTRETA, DisciNiscA and Schizocrania. A serious difficulty may sometimes 

 arise in the determination of the character of the pedicle-groove outside or 

 behind the aperture. By examining a specimen of the recent Discinisca lamel- 

 losa, it will be seen that the callous margins of the groove in this part of the 

 shell are connected by only a thin transparent lamella, which, if broken, as is 

 often the case, would leave the valve with a slit extending to the margin, as in 

 Schizocrania or Trematis. In the fossils under discussion, there sometimes 

 appears evidence that the sides of the groove are thus separated and not infre- 

 quently the pedicle-valve has been so figured, with a slit extending quite to 

 the margin. 



In certain species we have evidence that in the earlier stages of growth the 

 pedicle-groove opens upon the margin, its distal edges gradually approaching 

 as growth advances, until, at maturity, they are united, and with further in- 

 crease in age, Ihe distance of the groove from the margin is augmented. No 

 adult example, however, presents any other condition of this part of the shell 

 than that above described, except when a matter of imperfect preservation. 

 We have axamined a fragment of bituminous slate, from the lower Coal Measures 

 at Springfield, Illinois, bearing a greatnumber of individuals of the Dz^cma nitida 

 of Phillips (as identified by the Illinois palaeontologists), in various stages of 

 growth, from a diameter of .9 mm. to that of 9 mm., the latter being the size of 

 the average adult. A pedicle-valve .9 mm in diameter, shows that the margins of 

 the groove, from the beginning of the foramen outward, are quite widely separated, 

 but at a size of 5 mm. they have come into close approximation, without unit- 

 ing, while an individual of 5.5 mm. diameter has them distinctly united, but 

 in another, measuring 6.5 mm. diameter, they are still free. Larger examples, 

 in which the groove is wholly enclosed, bear a linear incision or track of the 

 groove quite to the margin. The same developmental process appears in Dis- 

 cina minuta. Hall, of the Marcellus shales, and D. Herzeri, of the Cuyahoga 

 shales, the younger stages of growth showing that the pedicle-groove is open 

 at the margin. 



