280 SYNOPTIC COLLECTION. 



A distinct line is seen in this figure marking off the 

 nepionic, Lingula-like stage from the succeeding neanic 

 stage. When the brachial valve is thrown open (fig. 5), 

 the crura (cr), which have already begun to form, are seen 

 supporting the crown of tentacles. The changes which 

 take place in the external shell between the stage repre- 

 sented in figs. 5, 6, and the completed shell, fig. 7, were 

 not figured by Morse. The development of the cardinal 

 region of the pedicle valve, however, was given (PL 691, 

 figs. 1-4). The pedicle opening becomes more circular 

 in outline and truncates the beak of the pedicle valve 

 (compare figs. 1-4 in PI. 691 ; see also No. 692, T. crassei 

 Dvds.). The deltaria are small and concave, while the 

 teeth, which are prominent in the earlier stages (PI. 691, 

 figs. 1-3, t), are reduced in size in the adult (fig. 4, /). 

 The development of the brachial loop is shown in PI. 691, 

 figs. 5-8. Beginning as little swellings or processes (fig. 

 5, <r), the crura grow larger (fig. 6, c), reminding one 

 of these parts in Rhynchonella (see PI. 680; No. 678). 

 Then the loop begins to form (PI. 691, fig. 7), which is 

 completed in the adult (fig. 8). 



The development of the brachia in Terebratulina illus- 

 trates the development of these organs in the Telotremata 

 generally. The first three stages are similar to those of 

 Glottidia in the Atremata (see PI. 655, figs. 1-3). The 

 lophophore in the first stage is a simple crescent with few 

 tentacles (PI. 693, fig. i); in the second stage the tenta- 

 cles have increased on either side of the median line in 

 front of the mouth and have been pushed backward until a 

 complete ring has formed around the mouth (fig. 2). 

 Next, the anterior edge of this ring bends inward (fig. 3). 

 The development is carried still further in Terebratulina 

 by the formation of a median unpaired arm (fig. 4). Fig. 

 5, T. cancellata shows a well developed spiral arm between 

 the two lateral arms. 



Certain gerontic peculiarities are seen in Terebratulina 

 cancellata Koch (No. 694). The nepionic shells seen in 



