JURASSIC FOSSILS. , 351 



by a scarcely perceptible groove bordering- the body of the valve. Poste- 

 rior wing smaller, triangular,' a little obtuse at the outer upper angle, and 

 proportionally somewhat higher or longer on the lateral border than the 

 anterior wing. Surface of the left valve marked, except on the fold and 

 depression bordering the cardinal slopes, by strong, flattened radii, which 

 are variable in strength, size, and distance, and are separated by flattened 

 interspaces, usually of greater width than the ribs themselves. The sur- 

 face is also marked by closely- arranged, distinctly-elevated, regular, lam- 

 ellose, concentric lines, parallel to the outer margin, and coincident with the 

 lines of growth, and are slightly arched backwards in crossing the radii. 

 These latter markings also cover the surface of the folds of the cardinal 

 borders and the cardinal auriculations. 



A single right valve of small size, associated with a number of left 

 valves on the same block, has the same general form and the same fold-like 

 feature of the cardinal slope, but appears to be entirely destitute of the 

 radii on the body of the valve, and the concentric markings are also much 

 less distinct and regular than on the opposite valve. The anterior wing is 

 also very small and obtusely triangular, but may have been somewhat 

 mutilated. There can be little if any doubt, however, that this right valve 

 belongs to the same species. 



This species may be readily distinguished by the peculiar fold-like 

 feature of the cardinal slopes and the character of the concentric mark- 

 ings; being entirely different in these respects from any other of the genus 

 described from rocks of this age. 



Formation and locality. — In reddish and buff-colored argillaceous lime- 

 stones of Jurassic age, at T Springs, west of the Black Hills of Dakota. 



Genus CAMPTONECTES Agassiz. 

 CAMPTONECTES BELLISTEIATUS. 



Plate 4, figs. 6-11. 



Pecten bellistriatus Meek, Proe. Acad. Nat. Sci. Phil. July, 1860, p. 311. 

 Camptonectes bellistriatus M., Smithsonian Cat. N. A. Jurassic Foss., 1864. 

 Camptonectes bellistriatus M. & EL, Pal. TJpp. Missouri, p. 77. 



Shells large, circular or slightly oblate in outline ; valves depressed- 

 convex, or when united lenticular; the left valve perceptibly the most 



