BRACHIOPODA. 



115 



composed mainly of alternating layers of phosphate of 

 lime and chitinous material. Cambrian to present day. 



Lingulella. Separated from Lingula by the presence 

 of a distinct slit in the ventral valve, and of a hinge-area. 

 Cambrian to Ordovician. 



Discina. Shell sub-orbicular, surface smooth or 

 covered with striae of growth. Valves conical, almost 

 equal, the dorsal valve deeper than the ventral, the 

 summits of both sub-central. Animal attached by a 

 peduncle passing through a foramen just behind the 

 summit of the ventral valve. Four adductor impressions, 

 the anterior prominent, the posterior small and widely 

 separated. Cambrian to present day. 



Crania (fig. 35). Shell calcareous, quadrangular or 

 subcircular, smooth or furnished with radiating ribs, fixed 



A B 



FIG. 35. Crania anomala, Recent. A, interior of ventral valve; B, dorsal 

 valve, a, anterior adductors ; a', posterior adductors ; c, posterior 

 adjusters ; c', cardinal muscle ; r, o, central and external adjusters. 

 (From Woodward.) 



by the ventral valve. Ventral valve depressed-conical; 

 dorsal conical with a sub-central apex. Internal border 

 of both valves covered with coarse granulations. Two 

 pairs of well-marked adductor impressions in each valve 



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