120 BRACHIOPODA. 



mented with radiating ribs arid often with lines of growth. 

 Ventral valve depressed in front. Dorsal valve often 

 much inflated. Hinge-line curved, no area. A small 

 circular foramen, sometimes with a small deltidium often 

 concealed. Ventral valve with two strong crenulate teeth, 

 muscular impressions grouped in the centre of the valve. 

 Brachial skeleton in the form of two spirals with their 

 apices directed toward the centre of the dorsal valve. 

 Ordovician to Trias. 



Rhynchonella (fig. 40). Shell triangular, elongated 

 or rounded, very convex, ornamented with radiating 

 ribs. A sinus on the ventral valve and a corresponding 

 ridge on the dorsal. Umbone small, acute, recurved; 



A B 



FIG. 40. Rhynchonella acuminata, Carboniferous Limestone. Internal 

 casts. V, 'vascular' impressions; 0, ovarian impressions; A, adduc- 

 tors ; B, divaricators ; P, muscles of the peduncle. (From Wood- 

 ward.) 



foramen enclosed by deltidium. Ventral valve with two 

 strong teeth, muscular impression in middle of valve. 

 Brachial skeleton reduced to two short, free, curved, 

 lamellae. Ordovician to present day. 



