C. E. Beecher — Development of the Brachiopoda. 143 



Thecidium (Lacazella) mediterranean Risso. 



Figure 17. — Cephalula; dorsal side, ds, dorsal shell plate; h, head. (After 



Kovalevski.) 

 Figure 18. — Dorso-ventral longitudinal section of cephalula of about same age 



as preceding, h, head; d, dorsal mantle lobe; v, ventral mantle lobe ; ds, 



beginning of dorsal valve; del, shell plate forming on dorsal side of body; p, 



pedicle. (After Kovalevski.) 

 Figure 19. — Typembryo. Larva transformed from folding upwards of mantle 



lobes, h, head ; ds, dorsal valve ; hi, hinge line of dorsal valve ; del, shell 



plate on body and pedicle posterior to hinge line of dorsal valve. (After 



Kovalevski ) 

 Figure 20. — Dorso-ventral longitudinal section of preceding. Keferences as in 



figure 19. vs, ventral valve. 

 Figure 21. — Profile view of nealogic Leplcena rlioniboidalis. The features of the 



shell are placed and lettered as in figure 20. ds, dorsal valve; hi, hinge 



line; del, deltidium: p, pedicle opening; vs, ventral valve. 

 Figure 22. — Adult Thecidium {Lacazella) mediterraneum ; dorsal side; showing 



ventral area and deltidium. 

 Figure 23. — Profile of same. References as in figures 20 and 21. 



at ds. In the process of transformation (figs. 19, 20), the man- 

 tle lobe is turned forwards in the usual manner, bringing the 

 shell on the outside of the animal, so that both dorsal plates 

 are now exposed, ds being the dorsal valve, and del the shell 

 developed on the dorsal side of the walls of the body, and 

 caudal segments. As this plate (del) is below or posterior to 

 the hinge line (hi), and extends down over the pedicle, it is 

 evidently the beginning of the deltidium. At the same time, 

 there is an extension of the edges of the mantle and pedicle 

 on the ventral, or lower side, and shelly matter is deposited, 

 forming the ventral valve (vs), figure 20. At this stage the 

 hinge line, figures 19, 20, hi, is the line between the dorsal 

 mantle shell (ds) and the dorsal body shell plate (del). The 

 beak of the ventral valve is separated from the dorsal beak by 

 the pedicle and the shell covering to the pedicle and body 



