426 MISSISSIPPIAN BRACHIOPODA 



Remarks. This species differs from A. minuta in its larger size, its 

 proportionally much less convex pedicle valve, and in the absence of 

 the minutely spinose surface markings. It is more nearly like A. parva, 

 but differs from that species in its larger size, in the more obtuse beak 

 of the pedicle valve, and in the more conspicuous development of the 

 concentric lines of growth. 



Horizon. Kinderhook (Rockford limestone). 



AMBOCCELIA LEVICULA Rowley 

 Plate LXXVII, Figs. 26-31 



1900. Ambocoelia levicula Rowley, Am. Geol., vol. 25, p. 262, pi. 5, figs. 



12-14. 



1911. Ambocalia levicula ? Girty, Bull. U. S. Geol. Survey, No. 439, p. 73, 

 pi. 8, figs. 7-9. 



Description. Shell small, broader than long, the greatest width near 

 the mid-length of the shell, the hinge-line much shorter than the greatest 

 width, the cardinal extremities rounded. The dimensions of a very 

 perfect example, a metatype, are : length of pedicle valve 5 mm., length 

 of brachial valve 4.1 mm., greatest width 5.6 mm., thickness 3.4 mm., 

 length of hinge-line 3.5 mm. 



Pedicle valve strongly convex, most prominent in the umbonal region, 

 the surface curving abruptly to the cardinal margin on either side of 

 the beak, and more gently to the lateral and anterior margins; the 

 mesial sinus faint, very narrow and shallow, originating at or near the 

 beak, becoming less well defined anteriorly; the beak prominent and 

 strongly incurved; cardinal area small, concave with the curvature in- 

 creasing towards the beak, differentiated from the lateral slopes of the 

 valve along its lateral margins by a slight angulation of the surface 

 extending from either side of the beak to the cardinal extremities, the 

 delthyrium rather large, its base occupying nearly one-third of the entire 

 hinge-line. 



Brachial valve depressed convex, the greatest convexity posterior to the 

 middle, the surface somewhat compressed towards the cardinal ex- 

 tremities, gently convex to the antero-lateral and anterior margins : a, 

 mesial flattening of the valve originates in the umbonal reerion which 

 passes into a rapidly widening, but very shallow, ill-defined mesial sinus 

 anteriorly; the beak very small, scarcely or not at all incurved, project- 

 ing very slightly beyond the cardinal margin. 



Surface of both valves essentially smooth in the specimens examined, 

 with neither concentric nor radiating markings. 



Remarks. This species is most closely allied to A. parva from the North- 

 view sandstone, but differs from that species in being proportionally 

 wider, and in having a much fainter mesial sinus in the pedicle valve. 



Horizon. Burlington white chert. 



