182 CRETACEOUS PEKIOD. 



ventral margin, twenty-six millimeters ; greatest breadth, eighteen milli- 

 meters; thickness, sixteen millimeters. 



This species is especially distinguislied by its small size. Its other 

 more conspicuous specific characters are the small but prominent and 

 pointed beaks and subequal valves. From the young of J. prohlematictis, 

 the valves of which are also subequal, it differs in the character of the beaks 

 just mentioned, the much greater convexity of the valves, and other evidences 

 of mature growth. 



The collections contain quite a large number of examples of this neat 

 little species, both valves of which, in a majority of cases, are together in 

 their natural position. 



Position and locality.— Qtrsita of the Cretaceous period; Ojo del Piscado, 



New Mexico. 



Family PINNIDiE. • 



Genus PINNA Linnaeus, 1758. 



Pinna petrina White. 



Plate XIII, fig. 7 a and h. 



Pinna petrina White, 1874, Expl. & Surv. west 100th Merid., Prelim. Eep. Invert. 



Fo.ss., 24. 



Shell moderately large, broad, rather thick, rapidly expanding in height 

 as it increases in length ; dorsal margin concave ; ventral margin convex ; 

 a more or less strongly-raised carina extending from the beak to the poste- 

 rior margin, defining a prominent longitudinal angle along the median por- 

 tion of each valve, which is placed a little nearer the ventral than the dorsal 

 border; transverse section rhomboidal, the sides of the rhomb being 

 slightly convex ; posterior margin oblique with the axis of the shell, form- 

 ing a distinct but obtuse angle with the dorsal margin. The acute angle 

 which it would form with the ventral margin if continued all the way to it 

 in a direct course is abruptly rounded. 



Surface marked by strong, distinct lines of growth, which run obliquely 

 downward and backward in a nearly direct course from the dorsal margin, 

 across the mesial angle, to near the ventral margin, where they are abruptly 

 flexed forward, and blend with the ventral border. Crossing the lines of 

 growth upon the surface above the mesial angle, there are coarse but indis- 



