25 



and narrowed behind, a little more than twice as long as high. Valves 

 very convex, becoming inflated near the middle and on the umbones. 

 Beaks proportionally large, erect, strongly projecting above the hinge 

 line, and situated in front of the middle of the length ; anterior end 

 large and broadly rounded, somewhat excavated beneath the beaks; 

 posterior end narrowed, elongate, and very slightly recurved, and ob- 

 liquely truncate above the umbonal angle; basal line strongly convex, 

 more distinctly so just in front of the middle opposite the beaks, and 

 broadly constricted behind ; posterior umbonal ridge very faintly an- 

 gular, and the cardinal slope rather abrupt. 



Surface of the shell marked by a few concentric lines of growth, and 

 crossed on the anterior and middle parts of the shell by a few propor- 

 tionally strong, elevated, radiating ridges, to the number of eight or 

 nine, with wider flattened interspaces. 



The species is small, and being preserved in a friable sandstone, the 

 surface characters are not clearly defined, nor can any of the hinge or 

 any other internal features be seen. 



Formation and locality. In white sandstones of Jurassic age, in Red 

 Water Valley, Black Hills, Dakota: associated with Pseudomonotis 

 curta, Tancredia Warrenana, and other Jurassic fossils. 



FAM. GASTROCILEXIim 



GENUS SAXICAVA, Bettevue. 

 SAXICAVA JURASSICA, n. sp. 



Plate 5, figs. 25-30. 



Shell small, elougate-subcylindrical, with subparallel dorsal and basal 

 margins ; beaks approximate, distinct, situated near the anterior end 

 and flattened on the exterior surface ; umbonal ridge strongly marked 

 and subangular, placed behind the middle of the valve. Anterior end 

 narrowed and very slightly excavated below the beaks ; posterior ex- 

 tremity wider and rounded on the margin, the sides being laterally com- 

 pressed ; basal margin very gently concave in the middle of its length. 

 Body of the valves very broadly sulcated between the anterior end and 

 the umboual ridge, and somewhat inflated along the latter feature pos- 

 terior 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 strke 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. 



