10 PEOCEEDIJSTGS OF THE NATIONAL MUSEUM vol. 78 



metrically oval, transversely elongated, and shows traces of con- 

 centric lines. Other characters obliterated. 



Length, 25 millimeters; height, 16 millimeters; depth of crest, 2.5 

 millimeters. 



Various forms resemble this, but comparisons seem of little value. 

 No species of similar age is known in the interior region, however. 



Cat. No. 73769, U.S.N.M. 



Genus TELLINA Linnaeus 



TELLINA? species 



Plate 1, Figure 4 



A single specimen constituting a mold of the interior of a left 

 valve and preserving a little of the shell seems to belong to the genus 

 Tellina but preserves so few details as to make a definite assignment 

 difficult. 



Valve depressed, in general transversely elongate-oval, with a pos- 

 terior basal angle, to which a faint ray extends from the low beak. 

 Anterior dorsal margin shorter than posterior dorsal margin, both 

 nearly straight ; basal margin gently convex ; anterior margin evenly 

 rounded ; posterior margin oblique, straight. Sculpture visible is of 

 fine concentric lines. Other characters not shown. 



Length, 19 millimeters; height, 12 millimeters; depth, 2 milli- 

 meters. 



No form much like this and of comparable age has been described 

 from the interior region. 



Cat. No. 73770, U.S.N.M. 



Family MACTRIDAE Gray 

 Genus MACTRA Linnaeus 



MACTRA? INCOMPTA (White)? 



Plate 1, Figures 12, 13 



Three molds of the exterior of valves are very much like Mactra? 

 irtcotJipta (White) ^'^ in size and form, differing only on the finer and 

 less regular concentric sculpture. Though this difference may be 

 due only to the manner of preservation it seems best to question the 

 identification of the material in hand until better specimens are 

 available. 



The valves are ovate-subtrigonal in outline, moderately convex; 

 beaks about one-third the length behind the anterior margin, only 

 moderately prominent as preserved; angle at the beaks a trifle more 



^« White, C. A., The invertebrate fossils collected in portions of Nevada, Utah, Colorado, 

 New Mexico, and Arizona : U. S. Geog. Surveys W. 100th Mer. Rept., vol. 4, pt. 1, p. 185, 

 pi. 17, flg. 6, 1877. 



