CLASS BRACHIOPODA. 



GENUS RHYNCHONELLA, Fischer, 1809. 

 RHYNCHONELLA GNATHOPHORA, Meek. 



Plate 8, Fig. 1, and 1 a, b, c, d, e,f. 



SHELL of medium size, subtrigonal or suborbicular, generally 

 rather compressed, but sometimes gibbous in adult specimens ; 

 the greatest convexity being near the middle. Postero-lateral 

 margins straight and diverging from the beaks at an angle of 

 from eighty to one hundred degrees. Dorsal valve usually more 

 convex than the other, particularly towards the front, where it 

 rises into a low, rounded, mesial prominence, rarely traceable 

 more than half-way back towards the incurved beak. Ventral 

 valve with a moderately deep mesial sinus, occupying about half 

 its entire breadth at the front, but narrowing and becoming more 

 shallow as it recedes, so as to die out near, or a little beyond, the 

 middle. Beak pointed, rather prominent, and moderately arched. 

 Surface of each valve ornamented by sixteen to twenty simple, 

 rounded, or subangular, radiating plications; three to four of 

 which occupy the sinus of the ventral valve$ and four to five the 

 corresponding prominence of the dorsal valve. 



Length (of a rather narrow specimen), 0.80 inch ; breadth of 

 do., 0.72 inch; convexity, 0.44 inch. Length (of wide compressed 



(39) 



