241 



AMBROC^LIA Hall (1860). 



AMBROCyELIA PLANOCONYEXA Shumard. 

 Plate, Figs. 9 a, b,c. 



"WTiite's description*: "Shell very small; breadth varying from a little 

 rmore to a little less than the length; hinge-line of considerable length, 

 but always shorter than the full width of the shell in front of it; lateral 

 4ind front borders regularly and continuously rounded. 



The dorsal valve would be almost circular but for its truncation by 

 the hinge-line; nearly flat, but slightly convex at the umbo, and sometimes 

 :slightly concave at the front; beak minute, not prominent; area very 

 marrow. 



Ventral valve capacious, especially its posterior portion, which extends 

 xnuch behind the hinge-line, and ends in a prominent strongly incurving 

 pointed bealv; area very narrow, high, concave, mesial sinus absent, but 

 in its place there is usually a slight flattening at the front and sometimes 

 an indistinctly impressed line is to be seen extending from beak to front. 



Surface apparently smooth, but under a lens it is seen to be finely 

 granular, tlie apparent granules being the bases of minute strife; a few 

 •concentric lines of growth are observable upon both valves." 



Position and Locality.— Strata of the Upper Red Wall, north bank of 

 ^Vhite River Canyon, twelve miles southwest of Fort Apache, Arizona. 



RETICULARIA McCoy (1844). 



RETICULARIA PERPLEXA. 

 Plate.FigB.lOa.b. 



Shell ordinary size, nearly circular in outline; breadth a little more and 

 'Convexity a little less than the length; hinge-line shorter than the full 

 width of the shell in front of it; lateral and front borders regularly and 

 ^continuously rounded; cardinal area distinct, arched, and moderately high. 



Ventral valve convex, extending much behind the hinge-line in a 

 prominent, strongly inciu-ved beak; area small; mesial sinus absent, but 

 in its place there is a slight flattening at the front and three indistinctly 

 impressed lines are to be seen extending from front to beak. This flatten- 

 ing gives to the shell a slight sinuosity. 



'■'White, U. S. Geog. Surv. W. of the 100th meridian, Vol. IV, P. 135, PI. 3, Figs. 10 a, b, c 

 16— A. OF Science, '03. 



