PELECYPODA—PRIONODESMA CEA . 



479 



Distinguished by its arcuate, subelliptical form and sinuate basal 

 margin. Growth lines lamellose. 

 Hamilton : New York, Ontario. 



LXXXVI. Unio Retzius. 

 Oval or elongated. Epidermis thin. Umbos more or less 

 anterior. Surface smooth, concentrically striated, corrugated, or 

 rarely spiny, Hinge line with heavy, amorphous, radial, pseudo- 



FlG. 640. a-c, Unio vetustus ; d, e, U. b elliptic alus ; f, g, U. dance ; h, U sen- 

 ectus ; i,j, U. holmesianus. (After White.) All X Yz- 



cardinal and lateral teeth. Ligament external. Pallial line simple. 

 Fresh water. Jurassic-Recent. 



308. U. vetustus Meek. (Fig. 640, a-c.) Cretacic. 

 Beaks marked with regular concentric ridges that usually end 



at the oblique umbonal ridge which extends from the umbo pos- 

 teriorly : a second ridge present between this and the cardinal 

 margin. Remainder of valve marked merely with growth lines. 

 Length about three inches. 



Bear River : Utah, Wyoming, Idaho. 



309. U. belliplicatus Meek. (Fig. 640, d, e.) Cretacic. 



