METAZOA VERMES. 279 



No. 685) belonging to the Jurassic, was of remarkably 

 large size. The pedicle opening and the deltaria are 

 well shown in the specimen. The anterior margin exhibits 

 the gerontic character of layers crowded closely together. 



The position of a living Terebratula is well shown in 

 No. 686, T. vitrea Born.; also No. 687, where the Brachi- 

 opod is attached to coral. The pedicle is tiny for the 

 size of the valves, which are nearly erect. The brachial 

 valve (No. 688, b) has the loop attached to it. This loop 

 began as a primitive Centronella loop and passed through 

 changes similar to those which will be described farther on 

 in the closely related genus Terebratulina. The pedicle 

 valve (No. 688, c) has a circular pedicle opening ; the del- 

 taria are small and concave, so that the brachial valve 

 moves upon the pedicle, though the motion is limited, 

 especially in the older, more rotund specimens. 



The development of Terebratulina septentrionalis Couth. 

 (PI. 689, figs. 1-7; No. 690), throws light on several 

 important points. The earliest stage observed after the 

 egg-stage is represented in fig. i, where the animal is 

 extremely minute, the length of its shell being indicated 

 by the line enclosed in the circle. It is attached to a 

 rock, and rests upon its broad hinge area with the anterior 

 margin uppermost, as seen in the drawing. The shell is 

 comparatively broad and short, and there is a wide pedi- 

 cle opening. Its form, however, changes rapidly, becom- 

 ing like Lingula (fig. 2), though still retaining the wide 

 anterior margin. 



The pedicle is long, allowing freedom of motion to the 

 valves which are seen in fig. 3 in a partial lateral view. 

 The animal is able to " whirl quickly " on its pivot-like 

 pedicle, and is represented in motion and at rest in fig. 4, 

 When in motion, it is nearly erect with the valves open, 

 and the cilia of the tentacles are active in catching food ; 

 while at rest, the valves are closed and the brachial valve 

 often lies on the rock or other object of support. Further 

 development causes the shell to broaden out anteriorly 

 and to become ornamented by ribs (fig. 6). 



