Middle Jurassic (Bajocian) Eastern Oregon

86 MIDDLE JURASSIC (BAJOCIAN) AMMONITES FROM EASTERN OREGON
Stephanoceras sp. A
Plate 45, figure 10
This species is represented by two laterally crushed
external molds. On the small septate whorls the primary
ribs are low, fine, closely spaced, incline strongly forward,
bifurcate a little below the line of involution and
are faintly to weakly tubercu]ate at the furcation points.
Adorally on the next to the largest whorl, the ribs become
abruptly stronger. The primary ribs become
moderate in height and spacing, divide near the
middle of the flanks into two or three much weaker
secondary ribs, and bear weak tubercles at the furcation
points. Most forked ribs are separated by single ribs that
arise freely near the middle of the flanks. The outermost
whorl, which may be part of the body chamber,
bears irregularly broad, low, flexuous striated ribs of
which some divide near the 1niddle of the flanks and
bear blunt tubercles at the furcation points.
Stephanoceras sp. A. is characterized by its forwardly
inclined primary ribs, by the fine and dense ribbing on
its sma1l whorls, and by the weakness of its lateral
tubercles.
Figured specimen.-USNM 168597.
Occurrence.-Snowshoe Formation undifferentiated
in the Senec.:t area at USGS Mesozoic loe. 25818. Possibly
represented also by .four small specimens at
Lupher's loe. 8.
Steph.anoceras sp. B
Plate 45, figure 9
This species, represented by one external mold, bears
forwardly inclined ribs as on Stephanoceras sp. A. It
differs by having much coarser and sparser ribbing, by
its outer whorl bearing many simple ribs, and by the
presence of lateral tubercles only on the outermost preserved
whorl.
Figured specimen.--CAS 13548.
Occurrence.-Snowshoe Formation undifferentiated
in the Seneca area at Lupher's loc. 57.
Stephanocer.as sp. C
Plate 44, figures 1-6
This species is represented by 25 molds of which most
are crushed. It has moderately involute coiling, a depressed
subquadrate 'vhorl section, a fairly high and
nearly vertical umbilical wall, weak lateral tubercles,
and high sharp wiry ribs that incline gently forward on
the flanks but cross the venter transversely. Most primary
ribs bifurcate at about two-fifths of the height of
the flanks into slightly weaker secondary ribs. Some
primary ribs remain simple and alternate with secondary
ribs that begin freely near the middle of the flanks.
Most furcation points are faintly swollen or · wettkly
tuberculate. Secondary ribs outnumber primary ribs
about 2=Y2 to 1.
The suture line has a broad first lateral s·addle, a long
slender trifid first lateral lobe that is as long as the
ventral lobe, a fairly broad and short second lateral lobe,
and a retracted suspensive lobe.
The specimen on which the suture line is exposed
appears to be only slightly compressed. At a diameter
of 61 mm, it has a whorl height of 22 mm, a whorl thickness
of 28 mm, and an umbilical width of 24 mm.
Stephanoceras sp. C differs from other species of
Stephanoceras from eastern Oregon in its subquadrate
whorl section, its fine, sharp ribs, and its weak lateral
tubercles. The characteristics of its ribbing suggest that
it may belong to Oadomites Munier-Chalmas (Arkeil
and others, 1957, p. L288) rather than Stephanocera8.
Figtrred speci1nens.-USNM 168598.
Ocew'1'ence8.-Snowshoe Formation undifferentiated
in the Seneca area at lTSGS Mesozoic loc. 29231; unnamed
beds in Juniper Mountains area, Malheur
County, at Mesozoic loc. 28649. Associated ammonites
indicate a stratigraphic. position in or near upper
Bajocian beds containing Spiroceras.
Stephanoceras sp. D
Plate 43, figures 10-14
This species is represented by 10 crushed molds that
show parts of five whorls. It is characterized by moderately
evolute coiling, by high, sharp, widely spaced
ribs, and by weak, radially elongate lateral tubercles at
the furcation points of the ribs. On the four smallest
preserved whorls the primary ribs incline backwards on
the umbilical wall and then incline gently forward to
the middle of the flanks where most of them divide into
pairs of slightly weaker secondary ribs. A few primary
ribs remain simple. A few secondary ribs arise freely
near the middle of the flanks. All secondary ribs incline
forward on the flanks and arch gently forward on the
venter. Secondary ribs outnumber primary ribs about
two toone.
The largest· preserved whorl bears very strong and
striated primary ribs, of which one forks near the middle
of the flanks and the others remain simple. Three of
the ribs bear weak lateral tubercles.
The species differs from Stephanoceras sp. A, described
herein, in having fewer, much stronger, and
more widely spaced primary ribs. The differences are
particularly striking on the small inner whorls.
Fig1.tred specimens.-USNM 168599, 168600.
Occurrences.-Snowshoe Formation undifferentiated
in the Seneca area at USGS Mesozoic loc. 28020. Unnamed
beds in Juniper Mountain area, Malheur County,