1 88 NORTH AMERICAN INDEX FOSSILS. 



ATREMATA. 



I. Obolella Billings. 



Small, thick- valved, oval or round shells. Narrow cardinal area 

 on each valve ; that on the pedicle valve is crossed by a pedicle 

 groove, not a slit. A long narrow muscular impression is present 

 on each side in the interior of both valves. Shell structure cal- 

 careous. Cambric-POrdovicic. 



I. 0. atlantica Walcott. (Fig. 222, a-d) Lower Cambric. 



Resembles O. crassa but differs in its average smaller size (yi-% 

 inch long), in its more circular form due to the lesser prominence 

 of the umbo and in its stronger concentric growth lines. 



Etcheminian of Massachusetts, Newfoundland. 





»*»! 



Fig. 222. a-d, Obolella atlantica; a, exterior of ventral valve; b, mold of 

 interior of same ; c, exterior of dorsal valve ; d, mold of interior of same (greatly en- 

 larged) ; e-g, Obolella crassa ; e, ventral valve, X 2 '■> /■> mold of interior of same ; g, 

 mold of interior of dorsal valve (a, cardinal ; c, central, and d, lateral or internal mus- 

 cular scars ; p, pedicle groove ; x, area) ; h, Obolella gemma, interior of a dorsal valve, 

 much enlarged. (All after Walcott. ) 



2. 0. crassa (Hall). (Fig. 222, e-g.) Lower Cambric. 



Sub-orbicular with beaks extending slightly beyond the margin. 

 The two valves are almost indistinguishable from each other ; the 

 beaks of both are curved down, that of the brachial valve nearly 

 touching the short, indistinct hinge line. Surface of both valves 

 radially and concentrically striated. Shell thick and solid. Average 

 shell is y^ inch in length. 



