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, althoag"!!;; 

 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 Idthodomus, 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 Bed Water Valley, Black 

 Hills, Dakota. 



