144 G E. JSeecher — Development of the Brachiopoda. 



lobe, or the deltidiura. The valves afterwards meet at their 

 peripheries, the hinge is extended beyond the deltidium, form- 

 ing the true hinge of articulate brachiopods. As there is no 

 motion between the ventral valve and deltidium, the two 

 become ankylpsed. Figures 22, 23, showing an adult Theci- 

 dium, are lettered in the same manner as the preceding, and 

 express the same relation of parts. 



24. 



26. 



28. 



-is 



Cistella neapolitana Scacehi. 



Figure 24. — Lateral view of completed cephalula stage, d, dorsal lobe of man- 

 tle; i', ventral lobe. (After Shipley.) 



Figure 25 — Dorso-ventral longitudinal section of same: showing posteriorly ex- 

 tended mantle lobes, ds and vs, inner surfaces of mantle lobes which are to 

 form dorsal and ventral valves. (After Shipley.) 



Figure 26. -Typembryo. Dorsal view of larva after transformation, h, head; 

 ds, dorsal valve; hi, hinge line of dorsal valve: p, pedicle. (After Kova- 

 levski.) 



Figure 27. — Dorso-ventral longitudinal section based on preceding ; showing man- 

 tle lobes directed forwards, bringing interior shell-secreting surfaces, ds and 

 vs of figure 25, on the exterior, h, head; ds, dorsal valve; hi, dorsal hinge : 

 vs, ventral valve; hi', ventral hinge; p, pedicle. 



Figure 28. — Dorsal view of early nepionic shell, showing large posterior open- 

 ing between valves. (After Kovalevski.) 



Figure 29. — Profile of same, ds, dorsal valve; vs, ventral valve; p, pedicle. 



The deltidium is not, therefore, primarily, on account of its 

 manner of origin, an integrant part of the ventral valve, but 

 is a shell growth from the dorsal side of the body, which after- 



