INVERTEBRATE PALAEONTOLOGY. 483 



least al this part, to come nearly or quite in contact, and leave an umbilical 

 cavity within the coil, almost equaling the greatest diameter of each surround- 

 ing whorl The surface is ornamented by distinct, oblique, annular costae, 

 which sometimes bifurcate on the lower outer side of the whorls, where they 

 support two rows of rather small nodes. The siphuncle occupies the middle 

 of the outside of the whorls, near the upper row of nodes. 



Although it is not possible to follow out the ramifications of the septa 

 in the specimen of this shell studied, they are evidently moderately distant, 

 and the principal lobes provided with long, very slender, branching, and 

 sharply digitate divisions. 



The whorl in our possession measures 3.27 inches across; the breadth 

 of the umbilicus being 1.10 inches; diameter at the larger extremity, 1.16 

 inches, and at smaller end, 1.01 inches. 



This form seems to be similar in some respects to our H. tortum ; but 

 the fact that the type-specimen (although larger, and consequently, if it 

 belongs to that species, would apparently represent a lower part of the spire) 

 has its volutions in contact, would seem to show that it cannot be identical 

 with that species. The only other shell I have yet seen among the Upper 

 Missouri collections resembling it in the size of the umbilicus compared 

 with the diameter of the whorls, is H. Nebrascense, from which it differs in 

 having much larger and more distant costae, and in being sinistral instead of 

 dextral. It also holds a lower geological position, where nearly all its associ- 

 ates, so far as known, differ from the species occurring with H. Nebrascense. 

 If a Heteroceras, our type-specimen must belong to the spiral part of a large 

 species 



Locality and position. — Same as last. 



Heteroceras? Cheyenne use, M. & H. 



Plate 21, figs. 2, a, b. 



Jncyloceraa 1 ! Cheyennense, Meek and Hayden (1856), Proceed. Acad. Nat. Sci. Philad., VIII, 71. 



TurriUtes Cheyennensis, Meek aud Haydeu (1856), ib., 280. 



Heteroceras^ Cheyennensc, Meek (1864), Smithsonian Check-List N. Am. Cret. Fossils, 25. 



The best specimen of this species we have obtained is a non-septate 

 fragment, about 2.50 inches in length, with a diameter of 1.55 inches. It is 

 a comparatively rather thick shell, with nearly cylindrical, disconnected 

 whorls, and forms a broad (dextral?) spiral curve in such a manner, that if 

 continued around, there would be left within the coil a great umbilical cavity, 

 apparently not less than four times the diameter of each surrounding volu- 



