296 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



shape and position of the adductor scars, and in the relative position 

 and arrangement of the cartilage-pits. It is less tortuous than G. tor- 

 tuosa Sowerby, and its proportions are different. The relation of this 

 species with G. subtortuosa is doubtless genetic, and it present<s one more 

 among other now known similar cases of e^ident genetic relationship 

 between the molluscan fauna of the Dakota Group and that of the later 

 Cretaceous groups of the West, which were formerly unknown, but which 

 the discoveries of Professor Mudge have done more than those of all 

 others to show. 



Position and locality. — Strata of the Dakota Group, Saline County, 

 Kansas, where it was discovered by Prof. B. F. Mudge, in whose honor 

 the sjiecific name is given. 



Genus PTBRIA Scopoli. 

 Subgenus Oxytoma Meek. 



Pteria (Oxytoma) salinensis (sp. nov.). Plate 5, figs. 1 and 2. 



Shell rather large for a Cretaceous Pteria; the body, exclusive of the 

 wings, obliquely subovate, broad at the base, moderately gibbous, dis- 

 tinctly but not very greatly inequivalve ; the left valve, as usual, more 

 convex than the right and its beak more prominent ; the convexity of 

 the valves somewhat uniform but increasing toward the umbonal region 

 in each, where it is greatest ; anterior wing moderately large, defined 

 from the body of the shell by being laterally compressed, but not by 

 any distinct auricular furrow; the byssal sinus under the anterior wing 

 of the right valve having the usual size and shape common to Oxytoma; 

 posterior wing not proportionally large, and not distinctly defined from 

 the body of the shell except by a somewhat gradual lateral compression; 

 its posterior angle not greatly ])roduced; hinge-line less than the axial 

 length of the shell; posterior adductor scars not distinct; anterior ad- 

 ductor scars distinct and deep for a shell of this genus, placed immedi- 

 ately in front of the beaks, that of the left valve being more distinct than 

 the other. 



This, like the last-described species, is known only by natural casts in 

 brown hematite of the interior of the shell, the imperfection of which 

 will not allow of an accurate measurement of all its proportions. It is, 

 however, known to have reached an axial length of more than CO milli- 

 meters, a transverse width near its base of at least 50 millimeters, and 

 a thickness of about 25 millimeters when both valves were in natural 

 position. 



The character of the surface is not known, but it was evidently nearly 

 smooth, as is usual with Oxytoma. It is related probably genetically to 

 P. {0.) nehrascana Evans & Shumard, but it is a larger and more robust 

 shell, with a proportionally larger anterior wing, more prominent beaks, 

 and broader base. 



Position and locality. — Strata of the Dakota Group, Saline County, 



