118 



BRACHIOPODA. 



sions, which are elongated, extending nearly two-thirds of 

 the length of the shell. Muscular impressions in the 

 ventral valve very small. Radiating vascular impressions. 

 Ordovician to Carboniferous ; perhaps also Lias. 



Orthis (fig. 37). Outline circular or quadrate. Both 

 valves convex, the dorsal sometimes flattened. Surface 

 radiately ribbed or striated. Hinge-line straight, generally 



A B 



FIG. 37. Orthis striatula, Devonian. A, interior of dorsal valve; 

 B, ventral valve, c, curved brachial processes (cruras) ; v, genital 

 impressions; h, area; t, teeth; a, adductors; d, divaricators. 

 (From Woodward.) 



shorter than the width of the shell. Area double, each 

 divided by a median triangular fissure. In the ventral 

 valve, two large teeth, supported by dental plates. Four 

 muscular impressions in the dorsal valve ; two long 

 narrow ones in the ventral. Cambrian to Carboniferous. 



Spirifer (fig. 38). Shell transverse, more or less 

 triangular, biconvex, smooth or ornamented with ribs or 

 striae. Often with a sinus on the ventral valve and a 

 ridge on the dorsal. Hinge-line long and straight. Area 

 double, that of the ventral valve triangular, often trans- 

 versely striated, divided by a median triangular slit. 

 Dorsal area small, also with a median slit. Teeth sup- 



