DEVELOPMENT OF THE BRACHIOPODA 259 



lobe is still directed posteriorly, as in the preceding figure, 

 and the underlying shell plate is shown at ds. In the process 

 of transformation (figures 103, 104), the mantle lobe is 

 turned forward 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 devel- 

 oped 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 (AZ), 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 101). At this stage 

 the hinge-line (figures 103, 104, W) is the line between the 

 dorsal mantle shell (ds) and the dorsal body shell plate (deV). 

 The beak of the ventral valve is separated from the dorsal 

 beak by the pedicle and the shell covering to the pedicle and 

 body lobe, or the deltidium. The valves afterward meet at 

 their peripheries ; the hinge is extended beyond the deltidium, 

 forming the true hinge of articulate brachiopods. As there 

 is no motion between the ventral valve and the deltidium, the 

 two become ankylosed. Figures 106 and 107, showing an 

 adult Thecidium^ are lettered in the same manner as the pre- 

 ceding, and express the same relation of parts. 



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

 its manner of origin, an integral part of the ventral valve, 

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

 afterward becomes attached to the ventral valve, and is then 

 considered as belonging to it. 



The further growth of the deltidium around the body and 

 pedicle, and its consequent extension into the cavity of the 

 ventral umbo, may explain the origin of the spondylium. 



K ovale vski ^^ believed the ventral valve in Tliecidium was 

 secreted by the expanded edges of the pedicle and the body 

 walls ; whether or not this is so does not affect the interpre- 

 tation of the origin of the deltidium. From the observations 

 of Lacaze-Duthiers,^^ it seems, however, as though the ven- 



