INVERTEBRATE PALAEONTOLOGY. 21 



names. It al least comes nearer the true G. vesicularis than any other form 

 vi -1 known from the Upper Missouri country. 



It differs from the prevailing forms of that species among the specimens 

 so common in New Jersey and Alabama; but we sometimes meet with indi- 

 viduals, in collections from New Jersey, almost exactly like our shell. It 

 resembles very closely forms referred by Groldfuss to G. vesicularis, especially 

 small specimens of the upper valve, but which M. d'Orbigny considers iden- 

 tical with Ostrea biauriculata-, Lam. 



Although resembling our last {O. patina), more than any other form 

 known from the Upper Missouri rocks, it is quite different from all the varie- 

 ties of that variable species I have seen; its under valve being much more 

 ventricose, and the scar of attachment left on its umbo much larger; while 

 there are indications of radiating lines on the smaller valve, which are never 

 seen on that of O. patina. 



Locality and position. — Cheyenne River, near the Black Hills; in the 

 Fort Pierre group of the Northwestern Cretaceous series 



ANOMIID^. 



Genus ANOMIA, Linnaeus. 



Synon.—Anomia, (part), Linn. (1767), Syst. Nat., cd. XII, 1150.— Miiller (1770), Zool. Dan. Prodr.,xxxi.— 

 Eetzins (1788), Dessert., 9.— Brug. (17§9), Encyc. Metb., I, 69.— Lamarck (1799), 

 Prodr. ; and (1801), Syst. Au., 137; also, (1809), PMIos. Zool., 317.— Eoissy (1805), 

 Moll., 239. — Defr. (1816), Diet. Sci. Nat., II, G6 : and numerous others. 

 Eckion, Poli (1791), Test. Utr. Sci., I. 34. 

 Echinoderma, Poli (1795), «Z>., II, 255. 

 Cepa, Humph. (1797), Mns. Col., 45. 



Fenestella, Bolten (1798), Mus. Boltenian. ed. 2d, 1819, 134 (not Lonsdale). 

 Anomya, Agassis (1839), Mould, de Moll., I, 23. 

 Etym. — nio/noc, unequal. 

 Examp. — Anomia ephippium, Linn. 



Shell orbicular or more or less irregular, very thin. Lower valve flat, or 

 modified by the surface of attachment, and having the sinus or aperture for 

 the passage of the byssal plug distinct, with the upper part of its anterior 

 lobe separated from, and often partly overlapping, the cardinal edge ; plug 

 thick, elongated, shelly, free from the margins of the aperture. Upper valve 

 convex, smooth, lamellar, costate, or marked by lines of growth; cartilage-pit 

 submarginal; muscular scars three, subcentral. 



So far as yet known, this genus seems to have had no existence anterior 

 to the Jurassic epoch It ranges through that and the succeeding forma- 



