MOLLUSCOIDEA—BRACHIOPODA. \ 7 i 



or dorsal. They are usually attached to some foreign object, such 

 as rocks, etc., by a fleshy stalk, the pedicle (Fig. 219, 5) ; this pos- 

 terior prolongation of the animal's body passes through an open- 

 ing, the delthyrium, or the more restricted foramen at the posterior 

 end of the pedicle valve. The shell is sometimes anchored by spines 

 and sometimes it is cemented to its support [Crania). 



9 



A 



FlG. 218. Diagram of Spirifer. {AB) longitudinal axis marking the height ; 

 {CD) transverse axis marking the width; [A) anterior (front) end; (B) posterior 

 (beak) end; (A) hinge line; (ca) cardinal area; (<?) cardinal extremities; (dt) del- 

 thyrium ; [u) umbo; {a) apex or beak. 



The interior of the shell is lined by the mantle, a membranous 

 reduplication of the body wall, which is often studded with minute 

 caeca or blind tubes which enter the perforations (tubules) of the 

 shell, thus giving the shell a punctate structure. The pedicle valve 

 is secreted by the ventral and the brachial valve by the dorsal 

 mantle lobe. In the Protremata the dorsal surface of the pedicle 

 secretes a third shellpiece, the deltidium, or pedicle plate, which 

 unites with the posterior margin of the pedicle valve and continues 

 to grow anteriorly {Schuchertella, Fig. 276). In the Telotremata, 

 where this deltidium is absent, the extension of the ventral mantle 

 lobe effects the protection of that portion of the pedicle by a secre- 

 tion of two plates, the deltidial plates or deltaria which may unite 

 to form the pseudodeltidium. In some of the Neotremata {Orbicu- 

 loidea, etc.), a single plate, the listrium, forms between the apex of 

 the pedicle valve and the opening for the pedicle. In some forms 

 [Spirifer) the delthyrium is filled by the deposition within the shell 

 of solid calcareous material. In extreme cases a tubular sheath, the 

 syrinx, is formed on this calcareous filling (Syringotkyris) (Fig. 436). 



Externally the shell is marked with concentric lines {growth 

 lines) which represent the successive stages of growth ; as the shell 

 grows a new layer is added to the inside, projecting beyond the pre- 

 ceding layer, thus forming a "series of outcrops." These growth 



