4 PROCEEDINGS OF THE NATIONAL. MUSEUM. vol. 65. 



Eutrephoceras sloani differs from E. coohana AVliitfield ^ chiefly 

 in size, the latter species attaining a diameter of 300 mm. The cross 

 section of the whorl is much the same in both species but is a little 

 broader in the Shark River species. It is possible that further col- 

 lecting will show that E. sloani attains a large size, has a considera- 

 ble range of variation in form, and is therefore inseparable from E 

 cookana. However, it seems best with the material now in hand to 

 consider the South Carolina species as distinct from the New Jersey 

 species. The other American species of the genus Eutrephoceras 

 known to the writer are UiDper Cretaceous in age and have nearly all 

 been referred at some time to E. dehayi (Morton), though unques- 

 tionably different, E. dehayi (Morton), as refigured by Whitfield," 

 from the Navesink and Redbank formations of New Jersey is about 

 the same size as E. sloani but is stouter, the proportion of height to 

 width of the cross section of the whorl being about 3 to 4. E. hryani 

 (Gabb), as refigured by Whitfield,^ from the Vincentown sand of 

 New Jersey, is more compressed than E. sloani^ the proportions of 

 height to width of the cross section of the whorl being about 8 to 7. 

 An undescribed species from the Ripley formation of Alabama is 

 stouter and apparently of consistently smaller size than E. sloani. 

 Another from the Navarro formation of Texas is separated by tha 

 presence of a persistent flattened zone on the venter. A third unde- 

 scribed species from the Gulf region occurs in the zone of Mortonice- 

 ras texanum and is stouter proportionately and much larger. A 

 species occurring in the upper part of the Pierre formation and 

 equivalent horizons in the Western Interior Province, called by 

 Meek ^ Nautilus dehayi Morton and forming tlie real type of the 

 genus Eutrephoceras^ is about the size of E. sloani but is much 

 stouter, the height to breadth of the cross section of the whorl being 

 about 3 to 4. An undescribed species from the Eagle sandstone of 

 Montana attains a larger size (240 mm. diameter) and is stouter, the 

 height to breadth of the cross section of the whorl being about as 

 6 to 7. 



EXPLANATION OF PLATES. 



Plate 1. 

 Eutrephoceras sloani Reeside, back view, five-sixths natural size. 



Plate 2. 

 Eutrephoceras sloani Reeside, front view, five-sixths natural view. 

 Plate 3. 

 Eutrephoceras sloani Reeside, side view, natural size. 



» Whitfield, R. P., U. S. Geol. Survey Mon. 18, pp. 285-286, 1892. 



• Whitfield, R. P., U. S. Geol. Survey Mon. 18, p. 243, pi. 37, figs. 1-6 ; pi. 38, figs. 1-4. 

 •» Whitfield, R. P., U. S. Geol. Survey Mon. 18, p. 244, pi. 38, figs. 5, 6. 



* Meek, F. B., Invertebrate Cretaceous and Tertiary fossils of the Upper Missouri 

 country: U. S. Geol. Survey Terr., vol. 9, pp. 49G-498, pi. 27, figs. 1. 2, 1876. 



