BRA CHIOPODA—A TREMA TA. 



189 



Etcheminian of Massachusetts, and Georgian of New York, 

 Quebec. 



3. 0. gemma Billings. (Fig. 222, h, 223, a, b.) Lower Cambric. 



Ovate. Pedicle valve with an acute beak. Brachial valve nearly 

 circular and obscurely triangular at beak. Surface of both valves 

 usually nearly smooth or feebly striate. 



Georgian of New York, Quebec. 



Average length ^ inch. 



4. 0. nitida Ford. (Fig. 223, c.) Lower Cambric. 

 Brachial valve gently convex. Hinge line slightly curved and 



equalling about one third the width of the shell. Surface marked 

 by a well defined median depression ex- 

 tending from the most elevated portion of 

 the valve forward for about one half the 

 length of the shell. Valve covered with 

 very fine radiating and concentric striae. 

 Diameter about y§ inch. 



Georgian of New York, and Etchemi- 

 nian of New Brunswick. 



II. Dicellomus Hall. 



Small, thick -valved, ovate, biconvex 

 with apices marginal. Outer surface of 

 shell finely punctate. Shell substance cal- 

 careo-corneous. Differs from Obolella in 

 the partially corneous character of the 

 shell substance, in the absence of a fora- 

 minal opening and in some internal char- 

 acters. In Dicellomus the interior of the 

 pedicle valve shows a short cardinal area 

 with a median pedicle groove, composite muscle scars and a short 

 shelf, like a spondylium extending into the valve from each side 

 of the pedicle groove. The interior of the brachial valve has a 

 narrow cardinal area in one species, D. politus. Middle-Upper 

 Cambric. 



5. D. politus Hall. (Fig. 224.) Middle-UpperCambric. 

 Valves moderately convex with greatest width near the front 



which is broadly rounded. Pedicle valve acuminate. Umbos 



Fig. 223. a, Obolella 

 gemma, ventral valve, X 6; 

 by dorsal valve, X 6 ; c , 

 O. nitida, dorsal valve, X 5 • 

 (After Walcott.) 



