538 



Forty-fifth Report on the State Museum. 



developed on the body of the thoracic lobe covered by this por- 

 tion of the mantle. Subsequently, during the neoembryonic 

 period, a shell-plate is also formed on the inner surface of the 

 ventral lobe of the mantle. By the inversion of the mantle- 

 lobes, these plates formed on their inner surfaces, become external 

 and inclose the cephalic lobe. Their attached or inner edges, 



Fig. 211. 



Fig. 213. 



Fig. 215. 



Fig. 212. 



Fig. 214. 



Fig. 216. 



Ciatella neapolitana. 

 Fig. 211.— The completed cephalula stage; the ventral and dorsal mantle lobes are at the right 



and left respectively. 

 Fig. 212.— Longitudinal section of the same; the shell secreting surfaces are represented by 



heavy lines. 

 Fig. 213.— Typembryo; the larva after the inversion of the mantle lobes. 

 Fig. 214.— Longitudinal section based on the preceding. The shell bearing surface of the mantle 



lobes are now on the outside of the animal, the large pedicle extending downward. 

 Figs. 215 and 216— Dorsal and profile views of a very young shell; showing the large posterior 



opening between the valves and the thick pedicle. (Bkkchkr, adapted from Kovalkvski 



and Shipley.) 



though separated by the entire width of the thoracic segment, 

 make the hinge-line. The body -plate is now in contact with the 

 dorsal-plate or valve at its inner edge, but does not touch the 

 ventral valve, the two latter being separated by the width of the 

 caudal Lobe or pedicle. This body-plate, or third valve, is 

 the incipient deltidium. Subsequently, by Lateral extension, the 

 dorsal and ventral valves come into contact at their cardinal 



90 



