JUEASSIC FOSSILS. 377 



flattened on the exterior surface ; umbonal ridge strongly marked and sub- 

 angular, placed behind the middle of the valve. Anterior end narrowed 

 and very slightly excavated below the beaks ; posterior extremity wider, 

 and rounded on the margin, the sides being laterally compressed ; basal 

 margin very gently concave in the middle of its length. Body of the valves 

 very broadly sulcated between the anterior end and the umbonal ridge, 

 and somewhat inflated along the latter feature posterior to the middle of 

 the length. Surface of the shell rather distinctly marked by concentric 

 lines parallel to the margin, which are elevated and irregular, but not 

 lamellose ; also by a few strong undulations of growth. Just below the 

 hinge line, on the postero-cardinal border, the surface striae are slightly bent, 

 and the surface of the shell very slightly angular. Internally, as seen on 

 casts, the anterior muscular impression is narrow, and situated along the 

 antero-basal margin. The pallial line appears to be strongly sinuate, and 

 the dorsal margin shows evidence of a rather strong ligament. The casts 

 of the burrows as seen in the rock are almond-shaped or very elongate- 

 ovate. 



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 eflbrt 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 material that the 

 description has been drawn. The general form of the shell and also the 

 external texture would seem to ally them with the angular group of the 

 genus Litlwdomus characterized by L. gruneri Phillippi, 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 Eedwater Valley, Black Hills. 



