JURASSIC FOSSILS. 353 



strong. It is possible that there may be several species confounded under 

 this name, but with the insufficient material yet brought in, it is impossible 

 to separate them satisfactorily. 



Formation and localities. — In rocks of Jurassic age, near Bear Lodge, on 

 the Belle Fourche River, 20 feet above the Red Beds ; on Spearfish Creek, 

 300 feet above the Red Beds ; in Redwater Valley, 350 feet above the Red 

 Beds ; and in the Big Horn Mountains. 



CAMPTONECTES EXTENUATUS. 



Plate 4, figs. 4, 5. 



Camptonectes extennatus M. & H., Proc. A. X. Sci. Phil., May, I860, p. 184. 

 Camptonectes extenuatus M. & H., Pal. Upp. Missouri, p. 78, PI. 3, fig. 6. 

 Camptonectes (?) pertenuis M. & H., ibid., Explan., PI. 3, fig. 6. 



Camptonectes (?) extenuatus (M. & H.) Meek, Smithsonian Check List Invert. Foss., p. — . 

 Camptonectes (?) extenuatus (M. & H.) Hall & Whitf., Geol. Expl. Fortieth Parallel, p. 

 290, PI. 7, fig. 18. 



Shell varying from small to rather above a medium size, somewhat 

 ovate or suboval in outline, exclusive of the auriculations. Hinge line of 

 the left valve straight, equal to a little more than half the greatest length of 

 the body of the shell in an antero-posterior direction; posterior side short, 

 but little more than half as long as the anterior. Auriculations large, tri- 

 angular, the posterior one obtusely angular behind, the margin sloping 

 rapidly backward to the junction with the body of the shell. Anterior 

 auriculation convex on the surface between the cardinal line and the body 

 of the shell below ; lateral margin nearly at right angles to the hinge, its 

 surface marked only by concentric lamellose lines. Body of the shell four- 

 fifths as great in an antero-posterior line as the height from hinge to base, 

 the anterior and posterior borders nearly equally rounded, the posterior 

 side forming a little the longest curve ; basal border more sharply curved. 

 Beak small, rather sharply rounded, and somewhat appressed, not projecting 

 above the hinge line. General surface of the valve strongly convex, most 

 ventricose between the beak and the middle of the valve, and in some 

 cases approaching angularity (perhaps from compression). Surface of the 

 shell marked by fine radiating striae, which diverge from the median line 

 and are strongly curved as they approach the lateral border, and still more- 

 23 BH 



