" INVERTEBRATE PALEONTOLOGY. 311 



Height, 0.91 inch; breadth, about 1.10 -inches ; heighl of aperture, 0.86 

 inch: breadth of same, 0.4.'! inch. 



This shell, judging from Mr Conrad's description of his Natica (Gyrodes) 

 crenata (of which no figure has yet been published), must be very closely 

 allied to that species, with which I had supposed it to be identical. On 

 sending the type-specimen, however, to Mr. Conrad, for comparison with his 

 species, he informed me that he thinks it clearly distinct; the lower carina 

 of his shell being much more strongly crenate, and the truncated upper edge 

 of its volution decidedly more distinctly wrinkled. As I yet only know a 

 single specimen of our species, however, it may possibly be found to vary in 

 these characters, so that no well-defined and constant differences can be found 

 on which to base a distinction. 



Locality and position. — Cheyenne River, Dakota Territory ; where it 



occurs associated with other forms believed to belong to the horizon of the 



Fort Benton group of the Upper Missouri Cretaceous. Collected by the 



late Dr. Hitz, of Washington City. 



Genus LUNATIA, Gray. 



Synon. — Naiiea and Ampullaria (sp.) of authors (not Natica, Adanson, nor Ampullaria, Lam.). 



Lutiatia, Gray (1»47), Proceed. Zool. Soc. Loud., IV, 149. — H. and A. Adams ( 1853), Genera Receut 

 Moll., II, 206 (as a genus). — Woodward (1856), Man. Moll., 123 (as a section of 

 Natica). — Chenu (1859), Mau. Couch., I, 214 (as a subgenus under Natica). — Meek and 

 Hayden (1860), Proceed. Acad. Nat. Sci. Philad., XII, 422 (as a subgenus under 

 Natica).— Gabl) (1861), Synops. Moll. Cret, Formation, 58 ; and (1864) PaL-eout. Cali- 

 fornia, I, 105 (as a genus). — Conrad (1863), Proceed. Acad. Nat. Sci. Philad., XV. 505 

 (as a subgenus under Natica); and (1865) Am. Jour. Conch., I, 23 (as a genus) ; also 

 (1873), Suppl. to Kerr's N. Carolina Geol. Report, 13 (as a genus). — Meek (1864), 

 Smithsouiau Check-List N. Am. Cret. Fossils, 20 (as a genus). — Tryon (1873), Ameri- 

 can Marine Conch., 59 (as a genus). 



Elym. — Luna, the moou ; from semilunar form of the aperture. 

 Type. — Natica ampullaria, Lamarck. 



Shell ovate-subglobose, rather thin, covered in recent examples with 

 a dull olivaceous epidermis ; spire more or less elevated ; aperture ovate- 

 semilunar; inner lip thin, or sometimes a little thickened above ; umbilicus 

 moderate or small, and without a callosity; operculum wholly corneous. 



Until comparatively recently, conchologists generally included this group 

 in the genus Natictt, either as a subgenus or without any distinction. The 

 most prevalent opinion, however, at this time, among the more reliable 

 authorities, is, that it is really a distinct genus from Natica, as properly 

 restricted to such forms as N. canrena, Linn., from which genus it differs not 

 only in its entirely corneous operculum, but in its more oval, usually thinner 



