DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 355 



specimen 1.5 mm. long, they first appear at about one-fifth 

 the length of the shell from the beak. Four plications are 

 included in the depression of the dorsal valve in the incipient 

 stages, and the two central ones finally become elevated, 

 forming the fold in the full-grown shell. Upon approaching 

 maturity, three of the plications in the ventral valve are 

 depressed, the middle one ultimately much more than the 

 others, forming the single strong plication at the bottom of 

 the sinus. 



No concentric striiB are shown on the initial shell of this 

 series. In a specimen o mm. in length, these begin to develop 

 over the outer third of the surface, as shown in figure 13, 

 Plate XVIII. 



Cardinal Area. — The foramen is at first a broad triangular 

 opening, wider than high, with sharp margins, and truncat- 

 ing the beak of the ventral valve. The lateral margins are 

 thickened in a specimen 3 mm. in length (Plate XVIII, 

 figure 13), and the height and width of the area are equal. 

 These proportions of height and width are preserved to 

 maturity, although in some specimens the area is higher than 

 wide. No deltidial plates have as yet appeared, but in the 

 next stage, including individuals having a length of 4.5 mm., 

 there are two narrow deltidial plates developed from the 

 sides of the foramen (Plate XVIII, figure 18). A specimen 

 5 mm. in length shows the still further increase in the size 

 of these plates, although they do not come in contact, but 

 leave an oval opening extending from the ventral beak down 

 to the beak of the dorsal valve (Plate XVIII, figure 19). 

 The increase in the growth of the deltidial plates along their 

 inner margins brings them in contact under the dorsal beak, 

 in specimens having a length of about 7 mm. (Plate XVIII, 

 figure 20). Further growth truncates their inner angles, 

 thus shortening the deltidial opening. In individuals about 

 12 mm. long (figure 21), the opening extends but little more 

 than half the length of the area, and the lower margin of the 

 opening is thickened and slightly deflected. Fully matured 

 forms, having a length of from 15 to 17 mm., have a perfora- 



