478 UNITED STATES GEOLOGICAL SURVEY OF ITIE TERRITORIES. 



The species here provisionally referred to this genus are only known in 

 the condition of mere fragments, evidently of spiral shells. Some of them 

 have the turns in contact, and some free. The former arc supposed to have 

 belonged to the spire, others to the part where the deflection was just com- 

 mencing, and still others to the deflected body-portion of the shell. All 

 would apparently go into the genus Turrilites as formerly understood, as well 

 as into the genus Helicoceras as extended by Mr. Sharpe and Dr. Stoliczka, 

 but not as originally founded by d'Orbigny. The changing and unsettled 

 state of opinion in regard to the limits of the genera Turrilites, Helicoceras, 

 and Heteroceras, together with the fragmentary condition of our specimens 

 of the Upper Missouri species, have caused their provisional reference at 

 different times in part to each of these genera. The weight of evidence, 

 however, seems to favor the conclusion that they belong to the genus Hetero- 

 ceras as now understood, though better specimens may yet show them to 

 belong wholly, or in part, to some other group. None of them have the 

 peculiar ornamentation of Anisoceras and Helicancyloceras. 



So far as yet known, the genus Heteroceras seems to be entirely confined 

 to the Cretaceous system. 



Heteroceras! c o c h I c a I u m , H. & M. 



Plate 22, figs. 2, a, b. 



Turrilites (Helicoceras) coehleatae, Meek and Haydeu (1858), Proceed. Acad. Nat. Sci. PLiilad., X,K>. 

 Helicoceras cochleatum, Meek (18t>4), Smithsonian Check-List N. Am. Cret. Fossils, 25. 



Shell sinistral, very thin, and composed of rounded, nearly or quite con- 

 tiguous whorls, which gradually increase in size from the smaller to the larger 

 extremity; umbilicus slightly wider than the largest whorl; surface orna- 

 mented by numerous rather regular, bifurcating annular costa?, which first 

 pass obliquely backward and outward from the umbilicus above, then curve 

 so as to cross the ventral or Outer side obliquely downward and forward ; but, 

 on reaching the under side, they curve backward again as they approach the 

 umbilicus. There are also two irregular rows of obscure, flattened, or de- 

 pressed oval nodes, one of which rows passes around nearly over the siphun- 

 cle, which is located near the middle of the outer side of the whorls, while 

 the other is placed less than one-fifth the circumference of the whorl lower. 



The septa are rather distant, and divided into complex lobes and sinuses, 

 which are a little unsymmetrical in their subordinate details, but about of 



