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. 



FlO. 252. 



FIG. 261. 



FlO. 267. 



FIO. SB. 



riG. 255. 



FIO. 256. 



Fio. 268. 



Thecidia (taeazeUa) Sfediterranea. 



Fig. 2.*»2. Cephalala, dorsal side; showing below, the cephalic segment with eye spots, and on the upper segment 



the dorsal shell -plate. 

 Fig. 253. Uorso-reatral longitudinal section of cephalala; 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 dcltidium. 

 rig. 2.^1. A later growth-stage, in which tlie mantle lobes have tamed downward. The body shell-plate is seen in 



the upper part of the flgurc. 

 FIf. 255 Dorso-ventral longitudinal section of the preceiling; showing the inversion of both mantle lobes. The 



relations of tlie dorsal ami body (dcltidium) plates are indicated by the heavy lines at the right. The 



Tentral plate is also seen on the lobe at the Icit. 

 ligs, 266. fronic of a very yonng Lrptrena rhomboiilalu, oriented to correspond with the foregoing flgures. 

 rigs. *i7, 25S. Views of adult Theculea Mediterranea, similarly placed. 



(Beecher; figs. 252-255, adapted from Kowalbvski.) 



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- 



