70 COLORADO FORMATION ANDMTS INVERTEBRATE FAUNA, [hull. 106. 



or only slightly convex; the remainder of the free border somewhat 

 regularly convex or rounded; lower valve broadly convex; hinge-teeth 

 well developed; ligamental fosset moderately large; upper valve nearly 

 flat or slightly concave in the rostral region. Surface of both valves 

 marked by small, slightly raised, radiating plications, which are crenn- 

 lated, a little irregular, increase in number both by bifurcation and 

 implantation, and are more or less distinct upon all parts of the surface 

 of both valves. 



" Length, 3 cra ; greatest breadth, 24 mm . 



" Position and locality. — Cretaceous strata, probably equivalent with 

 the lower portion of the Colorado Group; head of Water-pocket canyon, 

 southern Utah. 



" Collected by Mr. G. K. Gilbert." 



Only the original types, which are imperfect casts in sandstone, have 

 been found. It is possible that better collections would show that this 

 species is the same as Plicatula arenaria Meek, but with only the 

 imperfect material now in hand it is necessary to keep them separate. 



Plicatula arenaria Meek. 

 PI. ix, Figs. 3 and 4. 



Plicatula arenaria Meek, 1876, Macomb's Expedition from Santa Fe* to junction of 

 Grand and Green rivers, Geol. Rept., p. 126, PI. 1, Figs. 5 a, 6, and e. 



Original description : 



"Shell small, broad-ovate, usually a little oblique; ventral margin 

 rounded; sides converging to the beaks at an angle of about 70° to 80°; 

 beaks more or less angular. Under valve moderately convex. Upper 

 valve nearly or quite flat. Surface of each valve ornamented by eighteen 

 to twenty small, rather sharply elevated plications, only about half of 

 which extend to the beaks, while the intermediate ones usually extend 

 from one third to one-half way from the free margins; concentric mark- 

 ings rather obscure. 



"Length from beak to the most prominent part of ventral margin, 

 0.50 inch; transverse diameter, 0.43 inch; convexity, 0.11 inch. 



" Resembles, in size and form, P. incongrua, Conrad (United States 

 and Mexican Boundary Report, vol. i, PL 6, Fig. 10, 1857), but differs 

 in not having squamose concentric markings, as Avell as in having 

 shorter plications intercalated between those that extend to the umbo. 

 The substance of the shell must be thin, since the plications are rather 

 distinctly marked on internal casts. 



u Locality and position. — Covero [New Mexico]; Lower Cretaceous 

 of Dr. Newberry's section." 



