Logan.] The Invertebrates, Benton Group. 449 



obliquity ; basal outline very profoundly rounded, the deepest 

 or most prominent part being in advance of the middle ; poste- 

 rior margin moderately sinuous below the wing, from the ex- 

 tremity of which it ranges obliquely forward and downward, 

 rounded regularly into the base below ; anterior margin strongly 

 and subangularly sinuous under the wing, thence descending 

 with a slight forward obliquity and rounding rather abruptly 

 into the base ; hinge margin longer than the height of the valve, 

 the antero-posterior diameter of which (at any point below) it 

 also decidedly exceeds, ranging nearly at straight angles to the 

 vertical axis of the shell ; beaks distinctly convex, rising above 

 the hinge margin, strongly incurved, without obliquity, and 

 situated less than one-third the length of the hinge margin from 

 the extremity of the anterior wing, which is subtrigonal in form, 

 somewhat convex, a little rounded at the extremity, and very 

 strongly separated from the abrupt swell of the umbo by a deep 

 rounded concavity extending from the beak obliquely to the 

 marginal sinus below ; posterior wing longer and more com- 

 pressed, narrower, and more angular than the other; both 

 wings, particularly the posterior one, projecting decidedly be- 

 yond the margin of the valve below. Surface only showing 

 more or less distinct lines of growth. Height of left valve, one 

 and five-tenths inches ; length of same below the wings, about 

 one and three-tenths inches ; length of hinge line, one and nine- 

 tenths inches ; convexity of left valve alone, four-tenths inch" 

 i Meek). The types of Pteria (Oxytoma) erecia are small indi- 

 viduals that apparently belong to the same species. 



Two specimens, collected from a thin layer of sandstone 

 underlying the Lincoln marble horizon on Salt creek, in Mitchell 

 county, so closely resemble this species that I have referred 

 them to it, although the specific character of the wings can- 

 not be determined from the specimens which are not very 

 well preserved, especially in the umbonal region. A closer ex- 

 amination of this stratum will probably reveal more perfect speci- 

 mens. Should the specimens collected belong to this species 

 they are undoubtedly young individuals, as they are not more 

 than one-half the size of the above-described species. 



