172 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.xxii. 



ONCOCERAS ARCTICUM, new species. 

 (Plate XIV, figs. 4-7.) 



As shown by the figures, this species differs from all other American 

 Oncoceras in the great antero-j)osterior curvature of the ventral side of 

 the shell. This is caused by the rapidly increasing depth of the air 

 chambers toward the ventral side, where they are about twice the 

 depth of the dorsum. Siphuncle small, slightly constricted at each 

 septum, and closely adjoining the ventral wall except immediately 

 beneath the living chamber, where it is slightly deflected dorsally. 

 Venter more broadly rounded than the dorsum. Living chamber large, 

 with the sides slightly constricted, not less than 24 mm. deep, 33 mm. 

 dorso-ventrally, and 24 mm. transversely. The cast is smooth and pre- 

 serves no markings of the exterior. 



Cyrtoceras cornulum was first thought to be the young stage of 

 Oncoceras arcticum, but its section is more elongate-oval, with the dorsal 

 side more rounded, the reverse being true in the latter species. The 

 chambers also are deeper on the ventral side, the ventral curvature is 

 less strong, and the shell thicker, with longitudinal plications. 



Collector.— J. N. Oarpender. Cat. No. 2819G, U.S.N.M. 



ONCOCERAS TUMIDUM, new species. 

 (Plate XIV, figs. 1-3.) 



This species is much larger than any other in the Trenton of the 

 United States, although much smaller than 0. magnum, the giant of 

 the genus, which is found at East Selkirk, in Manitoba. The present 

 form seems to be most closely related to 0. gibbosum Whiteaves^ 

 (=0. whiteavesi Miller), but has not the numuloidal siphuncle of that 

 species, nor the sigmoid outline of the septa on the dorsum. The 

 living chamber, also, is shallower, being about 28 mm. deep. The con- 

 striction of 0. tumidum recalls that seen in Poterioceras or Gomphoceras, 

 but unlike these shells is not bilaterally symmetrical. The specimen 

 preserves but 6 septa and the living chamber, and throughout all 

 there is a rapid regular convergence toward the anterior eud. Dorsal 

 end of shell broadly rounded, with the ventral side somewliat acutely 

 rouuded. The 5 air chambers are dorsally 13 mm. deep and ventrally 

 25 mm. The siphuncle is small and situated about C mm. inside the 

 ventral wall. Septa regularly but deejjly concave. 



The type specimen was sent to Professor Whiteaves for comparison 

 with 0. ichiteavesi Miller. He reports that Oncoceras tumidum is "very 

 much like 0. tcMteavesi Miller { = 0. gibbosum Whiteaves, not Hall) 

 in lateral contour, bat in your specimen the body chamber does not 

 seem to narrow so abruptly as it always does in 0. whiteavesi, and shows 

 no sign of any constriction at the aperture." 



Collector.— J. K. Carpender. Cat. No. 28190, U.S.N.M. 



1 Trans. Royal Soc, Canada, Sec. 4, IV, 1889, p. 80, pi. xv, fig. 43. 



