BRACHIOPODA. 113 



interior is small. Sometimes there is a depression or 

 sulcus on one valve (generally the ventral) and a corre- 

 sponding ridge on the other valve. The surface of the 

 shell is sometimes quite smooth, or it may be punctate, 

 or ornamented with striae or ribs, which generally radiate 

 from the umbones, but are occasionally concentric. A 

 few forms possess spines. 



In the Articulata the shell is mainly calcareous. In 

 the genus Waldheimia it is formed of three layers (fig. 33), 

 the inner (a), next the mantle, is the thickest and most 



FIG. 33. Vertical section of shell of Waldheimia, flavescens, Eecent. 

 a, prismatic layer ; b, epidermal layer ; c, outer calcareous layer ; e, d, 

 canals traversing the calcareous layers. Magnified. (After King.) 



important, it consists of flattened calcareous prisms, 

 arranged obliquely to the surface of the shell, each prism 

 being encased in a chitinous membrane, which of course 

 has disappeared in the fossil examples. The middle layer 

 (c) is lamellated and also calcareous. The outer (6) is the 

 epidermal layer and consists of chitinous material. The 



FIG. 34. Horizontal section through the prismatic layer of Terebratula 

 maxillata, from the Great Oolite, showing prisms and canals. 

 Magnified. 

 W. P. 8 



