124 EXPLORING EXPEDITION PBOM SANTA EE 



same extent as 0. congesta of Conradj die specimens being usually found scattered 



about on the surfaces of calcareous and sandy slabs, to which, in most instances, they 

 were evidently attached during the life of the animal. 



Locality andpostiion. — Banks of the Canadian, Vado del Chama, Galisteo, Pagosa, 

 Middle Cretaceous of Dr. Newberry's section. 



OSTKKA (GBYPHJBAt) UNIPORMIS, .Meek. 

 Plate I, figs. 2«, b, c 



Shell small, rather thin, trigonal-ovate or subcircular in form, not oblique; under 



valve rather deep, subcarinate along the middle, and arched beneath from the beak to 

 the opposite margin; beak curved upward, but truncated by the area so as to rise little 

 above the margins; area small; margins on each sidv of the area sometimes faintly 

 marked by line erenulations alongthe groove for the reception of the edge of the upper 

 valve; surface marked by about three or four rather regular plications on each side of 

 the larger mesial fold or carina; lines of growth obscure; muscular scar transversely 

 oval, moderately distinct; upper value unknown. 



Length from the beak to the opposite extremity, 1.1(5 inches; antero-posterior 

 diameter, about 1 inch; convexity or depth of the under Naive, 0.65 inch. 



This IS a very peculiar shell, and cannot be confounded with any other species 

 with which I am acquainted. Its large, prominent, mesial fold, extending from near the 

 beak of the under valve to the opposite extremity, with three or four smaller plica- 

 tions on each side, give it much the appearance of some of the plicated BracUapoda 

 when viewed on the under side. The folds, or plications, vary somewhat on different 

 specimens; but usually they are remarkably uniform for a species of this genus, and 

 impart to the free margin opposite the beak a zigzag character, very similar to the front 

 of some species of Spirifer. No specimens of the upper valve were obtained; but it 

 will probably be found to have a large mesial sinus corresponding to the elevation of 

 the other valve, and must possess a projection, curving down at the middle of the free 

 margin opposite the beaks, to till the deep notch in the margin of the other valve. 



Thinking this species might be identical with one or the other of two peculiar 

 oysters described by Dr. Shumard, from the Cretaceous rocks of Texas, under the names 

 0. bcltapllca/a and 0. quadriplicate, I sent him sketches of our shell, and he wrote that 

 it is most nearly like his 0. beUqpUcata, though, he thinks, clearly distinct. The 0. hel- 

 laplicata attains a, much larger size, some of the specimens being as much as three inches 

 in diameter, while its hinge-line is proportionally much longer, and its surface distinctly 

 ornamented with concentric markings. 



Locality and position.— Pagosa; 'Middle Cretaceous of the New Mexican section. 



Genus EXOGYRA, Say. 



Exogyra columella, Meek. 



Plato I, iijjs. 3 a, b, c,d. 



Shell small, rather thin, ovate; posterior side forming a semi-oval curve from the 



umbo to the ventral edge; anterior side rounded below the beak; ventral mar-in 



