168 BRACHIOPODA 



from the Carboniferous Limestone, which has a breadth 

 of twelve inches ; the size of the shell in different genera 

 varies from this down to about a quarter of an inch. 

 Generally the shell is thin, but in some forms, such as 

 Productus (Daviesiella) llangollensis, it is thick and mas- 

 sive. The external form varies considerably; it may be 

 globular, ovoid, hemispherical, quadrilateral, or triangular. 

 Usually both valves are convex, but in some genera, one 

 is plane the other convex, or one may be concave and the 

 other convex ; in the last case the space in the interior is 

 often small. Sometimes there is a depression or sulcus on 

 the anterior part of one valve (generally the ventral) and a 

 corresponding ridge on the other valve, or there may be 

 two sulci and two ridges (biplication). The surface of the 

 shell is sometimes quite smooth, but is often ornamented 

 with striae or ribs, which generally radiate from the 

 umbones but are occasionally concentric. In a few forms 

 the shell is covered with spines. 



e d 



Fig. 73. Vertical section of shell of Magellania [ = Waldheimia) Jlavescens, 

 Kecent. a, prismatic layer ; b, chitinous layer ; c, outer calcareous 

 layer; e, d, canals traversing the calcareous layers. (After King.) 

 Magnified. 



In the Articulata the shell is mainly calcareous. In 

 the genus Magellania it is formed of three layers (fig. 73) ; 

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

 important, and consists of flattened prisms of calcite 

 arranged obliquely to the surface of the shell, each prism 



