BRACHIOPODA 163 



others (Spirifer, fig. 82) it is long and straight. The 

 posterior part of the shell is that near the hinge (fig. 70, a), 

 the anterior is the opposite margin (b). The length of the 

 shell is measured from the anterior to the posterior border 

 (J) — a ). The breadth is at right angles to this, from one 

 side of the shell to the other (c — d). The thickness is 

 measured from one valve to the other, perpendicular to the 

 length and breadth (e—f). In some genera (e.g. Terebra- 

 tula) the length is greater than the breadth, in others (e.g. 

 Strophomena) the breadth is greater. Between the hinge- 

 line and the umbo there is in some brachiopods (e.g. Cyrtia, 

 fig. 71) a flat or slightly concave portion of the shell, 

 usually triangular, on which 

 the ornamentation of the rest 

 of the shell is absent, the 

 surface being either smooth _. _ n _ Z~ t w 



° . m Fig. 71. Cyrtia exporrecta, Wen- 



or Striated ; this is known as lock Limestone. a, umbo of 

 ,i Ti. ventral valve : abc, area with 



the area. It may occur on deltidium in t ' he m ; ddle . & _ Cj 



both valves (e.g. Ortllis), but hinge-line. Natural size. 



is sometimes found on the ventral valve only. 



Nearly all living brachiopods are fixed to a rock or 

 other object ; but some fossil forms were free, especially in 

 old age (e.g. Productus). Some, like Crania, are attached 

 by the close adhesion of one valve to the rock ; others (e.g. 

 Strophalosia) by spines given off from the surface of the 

 shell. More commonly, however, the attachment takes 

 place by means of a stalk or peduncle ; this is a cylindrical 

 process, in some genera long, in others short, connected 

 with the mantle, and passing out either through an opening 

 in the ventral valve (fig. 72 A,/) or between the umbones 

 (e.g. Lingida, fig. 76). It is composed mainly of supporting- 

 tissue with a sheath of horny material, but in some forms 

 there are muscular layers also. In Lingida, which com- 



11—2 



