MESOZOIG FOSSILS. 



627 



Gallatin Valley, hills west of Snake River 4 miles south of second crossing, 

 and Cinnabar Mountain. 



The general custom :>f recent authors is followed in using the name 

 Pleuromya instead of Myaeites. 



Thracia weedi n. sp. 

 PI. LXXV. figs. 1-3. 



Shell of medium size, thin, compressed, elongate, subelliptical in out- 

 line; beaks rather prominent, submedian; dorsal margin sloping rather 

 rapidly and almost equally before and behind the beaks; anterior end 

 broadly rounded, most prominent below; posterior end subtruncate ; ventral 

 margin slightly convex, somewhat sinuous toward the posterior end; poste- 

 rior umbonal ridge narrow and sharply defined; surface marked by irreg- 

 ular concentric undulations and by numerous fine lines of growth. 



The specimens selected for illustration, which are of average size, have 

 the following dimensions, respectively: 



Length. 



Height. 



Convexity 

 of both 

 valves. 



mm. 



mm. 



mm. 



29 



19 



i 



26 



18 



5 



34 



20 



5 



All the examples in the collection have suffered more or less accidental 

 compression and distortion in the rocks, so that they show considerable vari- 

 ation in outline, and probably on account of this compression they do not 

 show the posterior gape that they should have if they really belong to the 

 genus Thracia. 



The species differs too much in outline and proportions from the two 

 forms of Thracia (?) described from the Jurassic of the Black Hills to 

 require detailed comparison. 



The types are from stream bed west of Little Quadrant Mountain 

 and from saddle in ridge west of south head of Gardiner River. Other 

 specimens were collected at head of Fawn Creek, northeast of Monument 

 Peak. 



