METAZOA VERMES. 271 



pedicle, but this notch becomes surrounded by shell lay- 

 ers in the adult. This mode of growth is also peculiar 

 to Discina, another member of the Neotremata, and when 

 it occurs in the more specialized groups of Brachiopods 

 it is usually called the Discinoid stage of development. 



One of the most instructive forms is Orbiculoidea (No. 

 658, O. lamellosa Hall ; No. 659, O. mimita Hall; PI. 660, 

 figs. 1-4, the same species; No. 661, O. nitida Phillips). 

 Beginning with the protegulum, it passes through the Pa- 

 terina stage (No. 659; PI. 660, fig. i) with its nearly 

 equal valves, straight external or marginal hinge line, 

 and its concentric parallel growth lines. The pedicle 

 passes out freely between the two valves. This is the 

 early nepionic stage. In the later stage, layers of shell 

 have encircled the protegulum (No. 659 ; PI. 660, fig. 2)> 

 causing the shell to become circular in outline, like Obo- 

 lella, with no hinge line, and with the beaks internal 

 though still near the margin. 



In the neanic stage the beak is still farther from the 

 margin (No. 659) while in the ephebic stage (No. 659) 

 it is slightly less eccentric and would seem to be passing 

 into the gerontic stage. The condition of the adult bra- 

 chial valve as compared with that of the young is shown 

 in PL 660, fig. 4. 



The pedicle valve (No. 658, a, b; PI. 660, fig. 3), has 

 the protegulum at the center and an open pedicle notch 

 as in the adult Paterula, but later the formation of circu- 

 lar layers of shell converts this notch into a hole through 

 which the pedicle passes (No. 658, a, b). This hole be- 

 comes partly filled with a plate called the listrium (see 

 No. 658, a, b). 



It is probable that Crania (No. 662, C. anomala-, No. 

 663, C. tubinata Poli) represents a reduced and specialized 

 condition of the Neotremata. The complete life history 

 of this genus has never been figured and described. It 

 is known that, when young, the pedicle valve has an open 

 notch like that of the adult Paterula. It is probable that 



