116 INVERTEBRATE PALEONTOLOGY. [WHITR. 



umbones broad; anterior end rounded or subtrimcate; base nearly straight 

 or very broadly convex, and often slightly emarginate about the middle; 

 postero-basal border rounded upward to the posterior extremity, which is 

 abruptly rounded to the downward sloping, nearly straight postero-dorsal 

 border, the latter forming an obtuse angle with the hinge border; hinge 

 equal in height to about two-thirds the entire length of the shell. 



A slight depression or flattening extends from the umbo of each valve 

 to its base, causing the straightening or slight emargination of the basal 

 border before mentioned. Area nearly obsolete ; hinge rather slender; two 

 or three long, slender transverse teeth occupy its middle portion; seven or 

 eight teeth cross the surface of its posterior end obliquely downward and 

 inward, and about an equal number of smaller ones cross the anterior end 

 almost vertically; the inner ones of the latter set of teeth being very small 

 and situated nearly beneath the beaks. 



Surface marked by ordinary lines of growth, and by fine radiating 

 lines, which are often obscure. Length, five centimeters; height, thirty- 

 three millimeters. 



Position and locality. Salt Wells Group; Coalville, Utah 



Genus UNIO Retzius. 



Unio yonionotus (sp. nov.). Shell elongate-subelliptical in marginal out- 

 line; flattened and thin when young, but becoming gibbous or almost cylin- 

 drical with age; dorsal margin broadly convex; base nearly straight; front 

 regularly rounded; the rounding of the posterior end somewhat irregular, 

 in consequence of the plications of the valves at that part; beaks obsolete, 

 the umbonal region of each valve so flattened that they form an acute angle 

 at the dorsum in the young, the angle increasing with age, so that it is very 

 obtuse in the adult shell. 



Surface of the anterior portion of the shell marked by only the ordi- 

 nary lines and lamellations of growth, but the posterior portion, comprising 

 more than half the length, is marked by strong, more or less irregularly 

 radiating plications, which begin faintly a little forward of the middle, and 

 increase gradually in strength to the posterior and postero-basal margins, 

 and increase in number by a few bifurcations toward those margins; curv- 



