TERTIAEY PERIOD. 207 



Cardinal tooth of right valve moderately strong, prominent, conical, 

 curved slightly upward and forward ; depressions upon each side of it 

 shallow; cardinal teeth of left valve not very prominent, nearly vertical ; 

 the pit between them for the reception of the tooth of the other valve rather 

 deep ; lateral teeth narrow, moderately prominent, very slightly curved ; 

 anterior adductor scars deep, placed close before and a little below the 

 cardinal teeth ; anterior pedal scars so close together as to form a single 

 depression, and placed immediately below the adductor scars. 



Some of the specimens in the collection are a little wider posteriorly 

 than anteriorly, and some are also more gibbous than others, which differ- 

 ences are thought to be sexual rather than varietal, since such sexual differ- 

 ences of form are usual among living species of Unto. 



Length, sixty-seven millimeters ; breadth, thirty -nine millimeters. These 

 measurements are of examples that appear to be of medium adult size. 

 Some fragments indicate that the species reaches a considerably larger size, 

 and also that some of them had a greater proportionate length. 



Mr. Meek has expressed the opinion, (Geological Survey of Wyoming 

 and contiguous Territories, p. 298,) that this species is identical with U. 

 prisons Meek and Hayden, but a still later development of facts leads me to 

 think the strata from which the two forms respectively come, belong to 

 different periods. Therefore I prefer to retain separate names for the two 

 fonns at present, although they are so similar. 



Position and locality. — Strata probably of Eocene Tertiary age ; Wales, 

 Utah. 



Family OYRENIDiE. 



Genus OYRENA Lamarck, 1818. 



Subgenus VELORITINA Meek, 1872. 



Cyrena (Veloritina) Durkeei Meek. 



Plate XXI, fig. 13 a and 6. 



Cyrena (Corhicula) DurJceei Meek, 1870, Proc. Amer. Philo. Soc. Phila., xi, 431. 

 Gorbicula {Veloritina) DurTceei Meek, 1871, Geol. Surv. Montana and adjacent Terri- 

 tories, 376. 

 Cyrena ( Veloritina) DurTceei Meek, 1874 (manuscript). 



Shell rather large for one of the genus, subtrihedral in outline, gibbous, 

 especially the upper median portion ; posterior and postero-ventral portions 



