266 SYNOPTIC COLLECTION. 



shell. The lines of growth run parallel with one another 

 and there is no ornamentation of any kind. The new 

 layers of shell are added at the anterior 1 and lateral 

 margins, so that the pedicle at the posterior end is always 

 free. 



The embryonic shell (uncolored in PL 647, figs, i, 2) 

 or protegulum (meaning early covering) is similar in form 

 to the mature shell. As we have already said, the youth- 

 ful (nepionic), adolescent (neanic), and mature (ephebic) 

 stages are all alike (see PI. 647, figs. i. 2), so that it is 

 impossible to tell where one stage ends or another begins. 



The valves of the shell are not articulated together by 

 teeth and sockets, but are held in place by muscles. The 

 pedicle valve is slightly more convex than the brachial 

 valve, so that an opening is produced through which 

 passes the short pedicle. This opening, according to 

 Beecher's figures, is shared alike by both valves, so that 

 the pedicle is free. As already stated, Walcott has found 

 the region between the two valves more or less closed by 

 rudimentary cardinal areas. 



Plate 648, figs. 1-4, represents Walcott's figures of 

 Iphidea. The brachial valve is seen in fig. i ; fig. 2 is 

 a summit view of the pedicle valve ; while a side view of 

 the same is shown in fig. 3. The imperfectly defined, 

 rather narrow cardinal area is shown in fig. 4 with a broad 

 plate, called the prodeltidium, just under the beak. The 

 latter is formed by the pedicle and not by the mantle as 

 is the case with the deltaria (see p. 277). 



The ancestral species from the Cambrian and Lower 

 Silurian, Obolella polita Hall (No. 649 ; PL 650, fig. i), 

 had a more or less circular form. This shape of the adult 

 shell is peculiar to many young Brachiopoda, so that when 



1 In order to see the characteristic parts plainly, the specimens are 

 mounted, and the figures are drawn, with the anterior end towards 

 the observer. This is contrary to the usual rule which places the 

 animal with the forward end away from the observer the most 

 favorable position when comparison with his own body is desired. 



