184 PALAEONTOLOGY OF CALIFORNIA. 



M. humerus, Con. 



(M. humerus, Con. Pacific R. R. Report, vol. 5, p. 321, pi. 2, fig. 10.) 



I am only acquainted with this species through the figure and description of 

 Mr. Conrad, quoted ahove. 



MODIOLA, Lam. 



M. SlSKIYOUENSIS, 11. S. 

 PL 30, Fig. 260. 



Shell long, narrow, subcompressed ; cardinal and basal margins 

 nearly straight, diverging posteriorly; beaks anterior, subter- 

 minal ; anterior end prominent, narrowly rounded ; posterior 

 very obliquely truncated above, rounded below. A well-marked, 

 rounded, umbonal ridge passes from the beaks to the posterior 

 basal angle. Surface marked by moderately distinct, concentric 

 ribs, most prominent posteriorly, and ending in fine lines in 

 advance. 



Figure, natural size. 



Localities : Siskiyou Mountains, California and Oregon ; and Jacksonville, Ore- 

 gon. Collection of the Survey and of Prof. Whitney. 



This species is of the type of M. concentrice-costellata, Roem., from Texas, but 

 is more compressed ; the anterior end is narrower and of a different form ; the 

 posterior end is more obliquely truncated, broader and more acute below ; and the 

 cardinal and basal margins are straighter. The ribs are smaller and more nume- 

 rous. 



M. ORNATA, 11. S. 

 PL 24, Fig. 166. 



Shell thin, broad, deep ; beaks small, anterior, subterminal ; 

 anterior end narrow, rounded, produced ; cardinal margin nearly 

 straight, uniting by a broad curve with the posterior extremity ; 

 anterior basal edge broadly emarginate ; umbonal ridge pro- 

 minent, rounded, curved downwards and widening out poste- 

 riorly, until it becomes lost in the general swell of the surface. 



