AMBOCCELIA 425 



differentiation; lateral slopes convex, smooth; mesial sinus originating 

 at the beak, narrow, angular in the bottom, not sharply defined laterally, 

 dividing the median flattened region of the valve along its middle line. 



Brachial valve not known. 



Remarks. The type specimens of this species are from a fine-grained 

 yellow sandstone from which all the shell substance has been removed. 

 The specimens, however, are not internal casts of the shell but apparently 

 retain the external configuration of the shell. The surface of the shell, 

 as preserved, is entirely smooth without lines of growth or other surface 

 ornamentation. The species differs from A. minuta in its larger size, 

 its less gibbous pedicle valve with the umbonal region projecting farther 

 posteriorly, and in the absence of the covering of fine surface spines. 



Horizon. Northview sandstone of the Kinderhook. 



AMBOCCELIA UNIONENSIS n. sp. 

 Plate LXXVII, Figs. 36-43 



Description. Shell small, wider than long, subelliptical in outline, the 

 hinge-line shorter than the greatest width, the cardinal extremities 

 rounded. The dimensions of two pedicle valves are : length 8.3 mm. 

 and 5.4 mm., width 10.3 mm. and 5.6 mm., convexity 3.6 mm. and 2.5 

 mm., length of hinge-line 7.5 mm. and 3.6 mm. 



Pedicle valve strongly convex, subhemispherical in form, the greatest 

 convexitjr near or posterior to the middle, the umbonal region broad and 

 projecting posteriorly beyond the hinge-line to only a moderate degree; 

 the surface curving abruptly from the umbonal region towards the car- 

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

 short, obtuse, only a little incurved; cardinal area of moderate height, 

 arched, the lateral margins slightly defined, sloping in nearly straight 

 or slightly concave lines from the beak to the cardinal extremities, the 

 delthyrium large, its base occupying one-third or more of the total hinge- 

 line; lateral slopes of the valve smooth, convex towards the middle of 

 the valve, becoming somewhat compressed towards the cardinal ex- 

 tremities in large individuals; mesial sinus originating at the beak, nar- 

 row, angular in the bottom, not sharply defined laterally. 



Brachial valve much less convex than the pedicle, the greatest depth 

 about half way between the middle of the valve and the posterior margin, 

 the surface compressed towards the cardinal extremities; the beak ob- 

 tusely pointed and extending a little beyond the hinge-line posteriorly; 

 the median portion of the valve depressed in a broad, shallow, ill- 

 defined sinus which originates near the beak and becomes rapidly broader 

 towards the front. 



The surface of both valves marked by usually well defined but not 

 always strong concentric lines of growth, aside from which the surface 

 is apparently unornamented. 



