JURASSIC FOSSILS. 357 



anterior side of the hinge. The wing is separated from the bod} r of the 

 shell below, as in others of this division of the group, by a narrow and 

 rather deep groove, but not by a distinct sinus. Posterior wing longer and 

 compressed, but not extending to more than half the length of the body of 

 the shell below. Surface of the valve marked, in all 'cases observed, by 

 fainter radii than the left valve, but otherwise of similar character. 



This species is closely related to P. (JE.) curta Hall, but is much larger, 

 several of the individuals measuring seven-eighths of an inch in length; 

 and one specimen measures nearly an inch, while those of that species 

 seldom attain to more than two-thirds of that size. The shell is also much 

 less oblique, less convex, the radii sharper and not usually so distinct, 

 although in this latter feature they both differ very materially among the 

 different individuals; and the right valve is also distinctly marked by the 

 radii, which is not the case in that one. 



Formation and locality. — In calcareous clays of Jurassic age, ranging 

 about 20 feet above the red beds, in Redwater Valley, Black Hills. 



Genus AVICULA Lam. 

 Subgenus OXYTOMA Meek. 



AYIOULA (OXYTOMA) MUCBONATA. 



Plate 4, figs. 1, 2. 



Pteria (Oxytoma) munsteri (Bronu) M. & H., Pal. Upp. Missouri, p. 80, f. A. B. 

 P. or Avicula (Oxytoma) mucronata M. & H., loc. tit., p. 81. 



Shell rather below the medium size, moderately oblique, the axis of 

 the shell forming an angle of a little more than 45° with the posterior 

 hinge line, although somewhat variable in this respect. Body, exclusive of 

 the wings, rather broadly and obliquely ovate, the greatest transverse diam- 

 eter being obliquely across the shell a little below the middle of the height. 

 Hinge-line very slightly declining on each side of the beak and somewhat 

 shorter than the length of the shell below; posterior wing on the left valve 

 about twice as long as the anterior, flattened or compressed on the surface 

 and mucronate at the extremity, separated from the body of the shell below 

 by a broad, but not very deep, sinus ; anterior wing small, slightly rounded 



