410 UNITED STATES GEOLOGICAL SUKVEY OF THE TERRITORIES. 



I know nothing of the septa of this shell, and, without seeing the other 

 extremity, it is impossible to determine whether it is an Ancyloceras or a 

 Samites. It may be at once distinguished, even in fragments, from any of the 

 forms we have referred to the genus Heteroceras, by being nearly straight, or 

 only bent in the form of a hook, instead of forming a spiral curve. Its larger 

 size and oval section would seem rather to favor the conclusion that it may 

 be an Ancyloceras ; but its position in the Upper Cretaceous renders this 

 doubtful. 



Locality and position. — South Fork of Cheyenne River, near the Black 

 Hills, where it was found in the Fort Pierre group of the Upper Missouri 

 Cretaceous series. It seems to be a rare species. 



PTYCHOCERATIDiE. 



Genus PTYCHOCERAS, d'Orbigny. 



Synon.—Ptychoceras, d'Orbigny (1841), Paleont. Fr. Terr. Cre"t., 1, 554.— Morris (1843), Brit. Foss., 184.— 

 Desbayes (1845), in Lamarck (2e e~d.),558.— Quenstedt (1852), Handb. Petref., 379.— 

 Meek and Hayden (1857), Proceed. Acad. Nat. Sci. Philad., IX, 134.— Cbeuu (1869), 

 Man. Conch., I, 95. — Meek (18G4), Smithsonian Check-List N. Am. Cret. Fossils, 23. — 

 Stoliczka (1866), Palaeont. Indica, 1, 193. 



T Soleiwceras, Conrad (1860), Jour. Acad. Nat. Sci. (n. s.), IV, 284. 



? Diptychoceras, Gabb (1869), Palteont. California, II, 143. 



Elym. — tttvxV, a fold ; KSpac, a horn. 

 Examp. — Ptychoceras Emericiamts, d'Orbigny. 



Shell slender, elongated, terete, at first straight, but at more advanced 

 stages of growth folded abruptly back upon itself once or twice, the different 

 limbs growing parallel and contiguous, or with the smaller slightly embraced 

 by the larger ; aperture oval or subcircular, with the inner side usually 

 rather sinuous; septa symmetrically divided into six generally nearly equal 

 lobes and sinuses, which are more or less branched and. digitate; siphun- 

 cle always on the ventral or outer side; surface of the young shell often 

 nearly smooth, but developing, at more advanced stages of growth, oblique or 

 transverse costse, which are strongest on the outer side, or entirely obsolete 

 on the inner or dorsal side. 



This genus seems to be most nearly allied to Hamites and Hamulina, 

 from both of which it differs in having its folded portions in contact with 

 each other, or slightly embracing, instead of free. Its relations to Hamites 

 are therefore precisely the same as the elongated forms of Scaphites to Ancy- 

 loceras ; in other words, it is a Hamites with the different limbs of the shell 

 compressed together. 



