MOLLUSCOIDEA — BRACHIOPODS 1 89 



their punctate appearance ; in some species, as Terebratella 

 plicata, this is quite coarse, being almost visible to the naked 

 eye. Within the mantle are hollow spaces, the pallial sinuses ; 

 impressions of these sinuses are often found upon the interior 

 of the valves. 



The pedicle valve is secreted by the ventral and the brachial 

 valve by the dorsal mantle lobe. In those forms in which 

 the brachial valve as a whole moves forward during the enlarge- 

 ment of the shell this would leave the dorsal surface of the pedicle 

 unprotected were it not that this surface has developed upon it 

 one or two special plates. In a portion of the Articulata (the 

 Protremata) the surface of the pedicle itself secretes a shell, — 

 the deltidium, which unites with the posterior margin of the 

 pedicle valve and continues to grow anteriorly (as in Rafines- 

 quina). In the rest of the Articulata (Telotremata) where the 

 deltidium is absent, the extension of the ventral mantle lobe 

 grows out laterally for the protection of that portion of the 

 pedicle by a secretion of two plates, — the deltidial plates (as in 

 Atrypa) . 



Teeth and sockets are absent in many forms with ceratin 

 (horn-like) shells ; these are therefore known as thelnarticulata, 

 the opening and closing of the valves being effected by a more 

 complicated set of muscles. In the great majority of brachio- 

 pods the shells are calcareous and here there are teeth and sock- 

 ets ; these are the Articulata. 



In the Inarticulata, where the setae upon the mantle margins 

 are long, acting as strainers (Fig. 75) to the ingoing water, the 

 shells are non-plicate. In the Articulata, where the setae are 

 short, the shells are often plicate. In a plicate shell the valves 

 need open but slightly to admit sufficient water and yet keep 

 out sand, enemies and other objectionable objects. 



The pedicle valve can usually be distinguished from the 

 brachial by some one of the following characters : 



1. Larger size and greater depth. 



2. Presence of a pedicle opening. 



