96 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



ous*in all cases. Although the substance of the shell is distinctly laminated, 

 the laminse appear obscurely fibrous on cross-fractures, when examined with 

 a magnifier. 



Associated with the above, a single valve of a much smaller individual 

 (figs. 14, b and c) was found, which has a comparatively smaller ligament-area, 

 a less prominent beak, straighter cardinal edge, and not so many hinge-teeth ; 

 while its costse seem to be obsolete over larger spaces at the extremities <>! 

 the hinge. This may possibly belong to another species; but as the differ- 

 ences noted may be due to the fact of its being a young shell, it is undesirable, 

 without more specimens for comparison, to separate it as a distinct species. 



Locality and position. — Heart River; upper part of the Fox Hills beds, 

 of the Upper Missouri Cretaceous series. 



Genus LIMOPSIS, Sassi. 



Synon.—Limopsis, Sassi (1827), Giron. Ligust. **.— Gray (1847), Zoiil. Proc, 198.— Meek and Hayd. (1856), 

 Proceed. Acad. Nat. Sei. Philad., VIII, 66.— H. and A. Adams (1858), Genera Recent 

 Moll., II, 543.— Chenu (1862), Man. Couch., II, 147.— Gabb (1864), Palajont. Cal., 1,200. 



Trigoiwcuelia, Kyst (1336), Eech. sur les coq. foss. de Houss., 12. — D'Orbigny (1843), Palaxmt. Fr., 

 Ill, 801. 



Trigonocelia, Sizmouda (1842), Synon. anim., invert., 20.— Pot. et Mich. (1844), Gall., I, 117. 



Trigonoaelia, Gray (1842), Synon. Brit. Mns., 92.— Nyst (1846), Foss. Tert. Belg., 293. 



Pectunculina, d'Orbigny (1843), Palasont. Fr., Ill, 182.— Geiuit-z (1846), Grundr. d. Ve.rst., 446. 



Etym. — Lima; and oftr, form. 

 Type, — L. multistriata, Forsk. 



Shell suborbicular, equiva've, nearly equilateral, sometimes slightly 

 oblique, closed all around; ligament-area small, with a triangular cartilage- 

 pit in the middle; hinge arcuate, and provided with a series of small, radiately- 

 arranged teeth in each valve, interrupted by the cartilage-pit; pallial line and 

 muscular impressions as in Axincea, excepting that the posterior muscular 

 scars are sometimes larger than the anterior ; surface concentrically or radi- 

 ately striated, or nearly smooth. 



The shells of this genus resemble Axincea, but may be readily distin- 

 guished by having a cartilage-pit at the middle of the hinge-area. They are 

 also usually smaller, more oblique shells. 



The genus Limopsis is said to be represented in the Alpine Trias. It 

 also occurs i-n the Jurassic rocks, as well as the succeeding formations, and in 

 the existing seas. Some twelve to fourteen recent species are already known, 

 and it is probable that the genus is quite as abundantly represented now as it 

 was at any of the pasl geological periods. The existing species occur in the 

 Chinese and Japanese Seas, the Red Sea, al the Cape of Good Hope, &c. 



