12G EXPLORING EXPEDITION. FROM SANTA FE 



Genus CAPROTINA, d'Orbigny. 



OAPEOTINA (EEQUIBNLA .') BIOOBNIS, "Meek. 

 Plate I, figs, ? <t, b. 



Shell apparently nearly equivalve; valves produced in the form of two 

 oblique spiral horns; beaks greatly elevated, distinctly spiral, and directed obliquely 



forward and outward, the turns of each being widely disconnected; aperture of 

 the valves comparatively small, ovate-subtrigonal in form, being a little higher than 

 wide, and nearly straight on the basal and anterior sides, which range at right angles 

 to each other; nmbonal ridges rather prominent, spiral, and curving down to the ante- 

 rio-basal margin. In front of these ridges, the valves are flat, and behind them convex. 

 Surface ornamented by moderately distinct marks of growth. 



Height, 2.43 inches; antero-posterinr diameter, 1.7-1 inches; transverse diameter 

 between the extremities ofthe beaks, nearly 2.75 inches; height of aperture of the valves, 

 1.38 inches; antero-posterior diameter of same, 1.22 inches. 



The only specimen of this curious fossil 1 have seen is imperfect, the beak of its 

 right valve being partly broken away. As nearas can be determined, its valves seem 

 to have been nearly equal, which, together with the greatly-produced and spiral char- 

 acter of its beaks, give it much the appearance of a Diceras. Still, as it shows no 

 traces of an external ligament, while its shell-structure and surface-marking are like 

 those of Caprot/ivi, I do not feel warranted in placing it in the genus Dicerus, though its 

 generic relations must remain doubtful until specimens showing the hinge and interior 

 can be seen. 



The substance ofthe shell is of moderate thickness, and composed of two layers, 

 which readily separate. The inner layer seems to be compact, and presents a distinctly- 

 striated surface, the stria' being parallel to each other and ranging at right angles to 

 the marks of growth. The outer layer appears to be more laminated, and breaks 

 readily at right angles to the surface'. Where portions ofthe shell have been removed, 

 a distinct groove; is seen extending from the posterior side of each valve spirally toward 

 the beak. 'These grooves appear to have been left by a ridge or lamina on the inner 

 side of each valve 1 , and curve around the beak so as to be visible in an anterior view, 

 as represented at (t, tig. 7 b, plate 1. 



In form and general appearance, this shell is evidently rather closely allied to Re- 

 quienia ArGhiaciana, d'Orbigny (Pal. Francais, Terr, ('ret., iv, 363; plate 597). The 

 grooves left upon the cast of its interior by the spiral ridge, or lamina, extending 

 from the posterior side of each valve, are also exactly as in that species. It differs, how- 

 ever, in having much less prominent and angular umbonal ridges, while its valves seem 

 to be more nearly equal, and its beaks are more produced and more widely separated. 



Locality and position. — Fort Lancaster, Texas; from the middle portion of Dr. 



Newberry's section. 



Genus PLICATULA, Lam. 



Plicatula ARENARIA, Aleck. 



Pluto I, li^s. 5 a, h, c. 



Shell small, broad-ovate, usually a little oblique; ventral margin rounded; .sides 

 converging to the beaks at an angle of about 70° to 80°; beaks more or less 



