66 UNITED STATES GEOLOGICAL SURVEY OE THE TERRITORIES. 



A side-view of this species is much like G. aviculoides of Sovverby 

 (Min. Conch., vol. 6, pi. 511), but it is a more arcuate and twisted shell, 

 though not so much so, nor apparently so obtuse at the anterior extremity, 

 as G. tortuosa of the same author. So far as is known, this and our 

 G. recta, and G. ensiformis of Conrad, from the Cretaceous of Alabama, are 

 the only species of this genus hitherto described from the rocks of this 

 country; the few species reported from Carboniferous rocks in the West by 

 Professor Cleinitz belonging, as already intimated, to Avicula, and possibly in 

 part to Bakevellia. 



Locality and position. — Three hundred miles above Fort Union, on the 

 Missouri River; in the Fort Pierre group, or No. 4 of the Cretaceous series 

 of that region. 



(i v r v ■ 1 1 i a recta, M & H. 



Plate 29, figs. 1, a, 6. 

 Gervillia recta, Meek and Haydeu (18fil), Proceed. Aead. Nat. Sci. Philad., XIII, 441. 



Shell small, rather thin, obliquely elongate-oblong, more or less arcuate 

 and nearly parallel on the lower and posterior dorsal margins, but not bent 

 laterally or twisted; anterior basal margin oblique, nearly straight, or form- 

 ing a long, slightly convex curve; posterior dorsal margin slightly concave in 

 outline; posterior extremity generally truncated and cuneate ; anterior extrem- 

 ity acutely angular ; hinge short, and ranging at an angle of 30° to 35° above 

 the oblique longitudinal axis of the shell; cardinal area a little gaping, pro- 

 vided with about three or tour small cartilage-pits; beaks small, very oblique, 

 placed near the anterior extremity of the hinge, but not terminal; surface 

 nearly smooth, or only marked with obscure striae of growth ; posterior alation 

 short and compressed ; anterior small. Left valve convex, but flattened 

 along the middle and behind, so as to impart a distinctly cuneate appearance 

 to the posterior end, and form an obscure, undefined ridge along near the 

 upper and lower magins of the flattened part. Right valve flat throughout. 



Length of a mature specimen, 2.10 inches; breadth at posterior end of 

 hinge, about 0.66 inch ; convexity, 0.30 inch ; length of hinge, 0.80 inch. 



This little Gervillia is so closely allied to a Lower Greensand species, 

 described by Professor Forbes under the name G. linguloides (Qr. Jour. Geol. 

 Soc. Loud., I, pi. .'!, tig. 9), that T have sometimes had doubts whether it is 

 really distinct. Still, a critical comparison shows that it differs in having its 



