64 UNITED STATES GEOLOGICAL SURVEY OF THE TEEKITOKIES. 



(•(/lured limestone layers, near the lower part of tlie Niobrara group, and 

 occasionally in the upper part of the Fort Benton group, of the Upper Mis- 

 souri Cretaceous series. I have never seen either from any localities east of 



the Mississippi. 



Genus GERVILLIA, Defrance. 



Synon.—Oervillia, Defrance (1820), Diet. Sci. Nat., XVIII, 51)2— Deslongehamps (1824), Mem. Soc. Linn. 

 tin Calvados, I, 120.— Blaiuv (1825), Malac, 530.— J. Sowerby ( 1826), Min. Couch., 

 VI, 16.— Desbayes (1830), Encyc. Mali.. Ill, Kifi.— G. B. Sowerby, jr. (1842), Man. 

 Conch. (2d ed.), 153.— Morris (1842), Brit. Foss., 108— d'Orbgny (1843), Palieont. Fr., 

 III, 480.— Geinitz (1846), Grnnd. d. Verst., 459 (not 1866, Carb. und Dyas in Nebraska). 



Gervillea, Fleming (1828), Brit. An., 381, and 390. 



Qervilleia, Roniinger (1846), Leoub. et Bronn's Jahrb., 296. 



Etijm, — Dedicated to the French naturalist Gerville. 

 Type. — Gervillia solenoidcs, Defr. 



Shell obliquely-elongated, ensiform or subtrigonal, very inequilateral ; 

 beaks oblique and placed near or at the anterior extremity ; left valve more 

 convex than the other; anterior margins a little gaping, but without a byssal 

 emargination in either valve : cardinal margin straight, more or less extended, 

 and somewhat alate posteriorly, and less so in front, provided with a flat, 

 diverging, or gaping area, or hinge-plate, crossed in each valve by from three 

 to about six distinct pits for the reception of the cartilage, which was exposed 

 externally ; hinge generally with a more or less numerous series of irregular 

 fold-like teeth under the area, ranging obliquely forward and upward; mus- 

 cular and pallial impressions as in Avicula. 



The genus Gervillia is closely related to Bakevellia, with which some 

 confound it. It differs, however, in generally having a larger, more elongated 

 shell, with usually a greater number of cartilage-pits, and more particularly 

 in having its hinge-teeth all ranging obliquely forward and upward, instead 

 of having those on the anterior and posterior sides elongated parallel to the 

 hinge-margin. Bakevellia, also, often has a larger anterior muscular scar. 

 From Melina, Retzius (= Perna, Bruguiere), the Gervillias differ in the 

 general possession of hinge-teeth, in being much more oblique, narrower, and 

 usually more elongated shells. 



As closely allied, however, as this genus is to Bakevellia, Dr. Stoliczka 

 refers that genus doubtfully to the Arcidce, while he places Gervillia correctly 

 in the ArkuUda-.* It seems to me, however, that they should go together, 

 wherever they are placed, and that Gervillia clearly belongs to the Avkulidce. 



* Pakcont. Iudieu, III, 335 and 392. 



