BRACHIOPODA. 
329 
body which subsequently becomes the pedicle. In this condition of growth the 
ventral lobe of the mantle is but slightly developed and bears no shell-plates. 
Fig. 253. 
FIG. 264. 
Fig. 255. Fig. 256. 
Thecidia (Lacazella) Mediterranea. 
Fig. 257. 
Fig. 258. 
Fig. 252. Ceplialula, dorsal side; showing below, the cephalic segment with eye spots, and on the upper segment 
the dorsal shell-plate. 
Fig. 253. Dorso-ventral longitudinal section of cephalula; below is the cephalic segment, at the right the dorsal 
mantle lobe, the darker line on its inner margin representing the beginning of the dorsal valve, and 
the similar line on the adjoining side of the body the incipient deltidium. 
Fig. 25t. .V later growth-stage, in which the mantle lobes have turned downward. The body shell-plate is seen in 
the upper part of the figure. 
Pig. 255. Dorso-ventral longitudinal section of the preceding; showing the inversion of both mantle lobes. The 
relations of the dorsal and body (deltidium) plates are indicated by the heav)^ lines at the right. The 
ventral plate is also seen on the lobe at the left. 
Figs. 256. Pro tile of a very young Leptrsna rhomboidalis, oriented to correspond with the foregoing figures. 
Figs. 257, 258. Views of adult Tliecidea Mediterranea, similarly placed. 
(Beecher; tigs. 252-255, adapted from Kowalevski.) 
These features are seen in the accompanying figure of a longitudinal section of 
such an embryo. In the directly following growth-stage the reversion of the 
mantle lobes has taken place; the shell-plate before on the inner surface of the 
dorsal lobe is now on its outer surface, and assumes the normal position of the 
dorsal or brachial valve. A corresponding plate has developed on the outer sur¬ 
face of the ventral mantle lobe, and between the inner edges of these two plates 
lies the great pedicle which bears on its dorsal side a third plate, meeting the 
dorsal, but widely separated from the ventral plate. This third plate is the 
incipient deltidium. The deltidium is, thus, not a secretion from the mantle, 
but from the body of the embryo, and it has been shown that the shell-puncta- 
