26 



A small fragment of argillaceous rock in the collection was found to 

 contain quite a number of the burrows, and, on being broken, a few 

 were found with the shells in position ; but in an effort to develop them 

 from the matrix, the shell is usually removed from the casts, although, 

 some of it is at times retained, and it is from this rather imperfect ma- 

 terial that the description has been drawn. The general form of the 

 shell and also the external texture would seem to ally them with the an- 

 gular group of the genus Lithodomus, characterized by L. gruneri Phil- 

 lipi, but the sinus of the pallial line would seem to forbid this reference. 



Formation and locality. In rocks of Jurassic age, at a horizon of 350 

 feet above the red beds of the Triassic, at Red Water Valley, Black 

 Hills, Dakota. 



