.TURASSIC FOSSILS. 357 



anterior side of the liinge. Tlie wing is separated from tlie bod}- of the 

 shell below, Jis in others of this division of the j^roup, by u narrow and 

 rather deep groove, but not by a distinet sinns. 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. (E.) curta TIall, but is nuich 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, Avhich 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. 

 AVICULA (OXYTOMA) MUCEONATA. 



Plate 4, fijjs. 1, 2. 



Pteria {Oxytoma) mvn.sieri (Broiiii) M. »& H., Pal. Upp. Missouri, p. 80, f. A. B. 

 P. or Avicula {Oxytoma) mucronaia ]\L & H., loc. cit., p. 81. 



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

 the shell forniing 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 extremit}', separated from the body of the shell below 

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



