16 AMERICAN JOUENAL 



tinuons and nearly equal curve throughout; cardinal margin 

 nearly straight and two-thirds as long as the width of the shell, 

 ending anteriorly by a slight angle ; posteriorly produced into 

 a prominent ear a little longer than high. The lateral face of 

 the ear is slightly emarginate below. Surface closely sculptured 

 by very faint, flat, radiating ribs separated by linear depressions, 

 the whole crossed by numerous minute and very regular con- 

 centric lines. Internally the radiating ribs are even more 

 strongly marked than on the surface. 



Length, 3-25 in., width, from beak to base, 3 inches, greatest 

 diameter of the two valves '8 in. 



Locality. Rare at New Pass, near Austin, Nev. ; from the 

 Trias ; found by Dr. Blatchley. 



This species resembles in outline 31. substriata, Munst., of the 

 Lias, but is much larger and even more round than that shell. 

 The surface is also entirely different. From 31. salinaria, Bronn, 

 and 31. sub-ci7'cularis, nobis, of the same formation in which it 

 is found, it is distinguished both by outline and surface ; though 

 in the presence of the anterior angle it approaches the former of 

 these. 



PECTEN, Brug. 



Three species of this genus are represented in the collection, 

 each by a single specimen. Two of these are flat and character- 

 ized by fine radiating ribs ; the third is convex, and the fragment- 

 ary internal cast resembles strikingly the deep valve of the Cre- 

 taceous genus Neithea. They are all from Volcano, the last 

 undoubtedly from the Jurassic. In regard to the age of the 

 others I am in doubt, whether to refer them to the Jura or Trias. 



P. acutipUcatus, Meek, is common at Volcano. 



PLICATULA, Lam. 



P. PERIMBRICATA, Gabb. — PL 6, fig. 15a. 



Shell of moderate size, inequivalve, more or less inequilateral, 

 sometimes slightly oblique ; sometimes free, sometimes apparent- 

 ly attached by the flat valve. Sides and base broadly and pretty 

 regularly rounded ; the right side sometimes a little more promi- 

 nent than the left ; this rounding continues from a half to two- 

 thirds the length of the shell, after which the sides rapidly con- 

 verge towards the beak. Surface of the upper valve convex, 

 and somewhat prominent in the middle, falling very gradually to 

 the base and convex portions of sides, more rapidly towards the 

 beak and superior lateral margins. Both valves marked by 



