241 



AMBR0C.5::LIA Hall (1860). 



AMBROC.ELIA PLANOCON^'EXA Slmmard. 

 Plate, Figs. 9 a, b,c. 



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

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

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

 and 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 

 narrow. 



Ventral valve capacious, especially its posterior portion, which extends 

 much l)ehiud the hinge-line, and ends in a prominent strongly incurving 

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

 in its place there is usually a slight flattening at tlie 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, the apparent granules being the bases of minute strife; a few 

 <!onceutric lines of growth are observable upon both valves." 



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

 White River Canyon, twelve miles southwest of Fort Apache, Arizona. 



RETICULARIA McCoy (1844). 



RETICULARIA PERPLEXA. 



Plate, Figs. 10 a, 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 incurved 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— .\.0F Science, '03. 



