164 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull. 106. 



" Length or greatest diameter, about 3.90 inches; height, 2.82 inches; 

 breadth at aperture, 3.40 inches." 



Locality and position. — Chippewa point on the upper Missouri river, 

 in Fort Benton shales. 



AMMONOIDEA. 



LYTOCERATID^E. 

 Genus HELICOOEEAS d> Orbigny. 



Helicoceras pariense White. 

 PI. xxxv, Figs. 2-4. 



Helicoceras pariense While, 1876, U. S. Geog. and Geol. Sur. West 100th Meridian, 



vol. iv, p. 203, PI. 19, Figs. 2a-d. 

 Compare Ancyhceras annulatus Shumard, 1860, Trans. St. Louis Acad. Sci., vol. I, 



p. 595. 



Original description: 



" Shell dextral; spire much depressed; whorls distinct, subcircular 

 or very broadly oval in transverse section, increasing somewhat rapidly 

 in size; surface marked by comparatively strong, rather abruptly 

 rounded anuulations, which cross the whorls obliquely; annulations 

 only slightly prominent upon the inner side of the whorls, but more 

 prominent upon the upper and under sides; upon the outer side of the 

 whorl each annulation bears a pair of prominent nodes, one on each 

 side of the siphuncle, forming two dorsal rows of nodes along the whole 

 length of the shell, the portion of the annulation between each pair of 

 nodes being straightened and slightly flattened upon the back. The 

 annulations are apparently always simple, never coalescing, and never 

 failing to completely encircle the volution. The nodes are moderately 

 prominent upon exfoliated specimens, and where the test is preserved 

 they are seen to be subspinous or sharply nodose. 



" Septa moderately distant, sometimes embracing two annulations, 

 but toward the aperture only one. Lobes all smaller than the saddles, 

 the size in each transverse series gradually diminishing from the dor- 

 sal to the ventral one; the smallest saddle, the ventral, not being 

 larger than the largest lobe, the dorsal; lobes all bifurcate, except the 

 ventral, the inferior lateral lobe being but slightly so; the anterior por- 

 tion of the space between the branches of the dorsal lobe occupied by 

 two backward projecting points; the ventral lobe is simple, small, nar- 

 row, and serrate upon both sides. The saddles of the different longi- 

 tudinal series all similar in shape, diminishing gradually in size from 

 the dorsal to the ventral series; all broader than long, except the ven- 

 tral one, the length and breadth of which are about equal; each par- 

 tially parted at the middle; edges of all the lobes and saddles serrated 

 or toothed. 



The longest fragment in the collection measures about 7 cm . At the 



