Publicaciones Geolo_gicas Especiales Del lngeominas

ZONATION OF THE CRETACEOUS OF CENTRAL COLOMBIA BY AMMONITES 27
Holotype. C-13-33 (U. N. C. M. H. N. C. P. Ht.
N.12).
Very similar to the previously described
species, but in the fanner the elements of the
suture line are higher and the saddles are more
finely zigzagged-lncised;"'in the preceding spe­
cies the adumbilical and central thirds of the
flank are Oat, the adventral third of the flank is
slightly beveled. In Car/oscaceresiceras monteroi
n. sp. the flanks are faintly convex and furthermore
they show more indication of ribbing on
the internal mold.
OCCURRENCE. Capotes Member-Lower
Albian.
This species is named in honor of geologist
Juan Montero:
loceresjcerns C? J £hjmuense, (Bena·
V1 es ace res)
Pl. 11, fig. 9; Text-figs. 4J, 4K.
Hypotype. VL.33a. (U.N.C.M.H.N.C.P. Hy. N.3).
1956 Desmoceras chimuense Benavides-Caceres,
p. 438, pl. 41, figs. 5·8.
1964 cf. Desmoceras chimuense Benavides·Ca·
ceres, Etayo-Sema p. 37, fig. 9.
One internal mold in black calcite. The in,
nennost whorl observed is higher than wide in
section, it has flat flanks which round abruptly
into the short but vertical umbilical wall. The
ventral zone is not discernible ; the visible part
of the flanks shows fine growth striae which
form .a shallow arc from the umbilical seam to
the involution line. The last whorl preserved
has flattened sides which feebly diverge from
the evenly arched venter; the section reaches its
m!D4mum width about the umbilical third of
the who.rl where the sides round abruptly into
the yertical umbilical wall. On the surface of
th .. is mold, vague
indications of constrictions are
Present but the exact trend of them can not be
.
aec.u:rately detennined because of the state of
prervation.
M.asurementl D Euw H W D{Euw % D/H %
VL"33;. at 7 19 33 30 25 . 44
.. . COMMENTS. In suture line and whorl
$ection this specimen resembles that described
by BenavidesCaceres as Desmoceras chimuense
Bnavides·Cce s, 1956, p. 43, pl. 41, fig. 5·
8, '!'he _spec1es IS referred tentatively to Carlos·
cQcereslceras ..
. ·. ·.· OCCURRENCE. Upper San Gil Forma·
tion, r.,evel 8. Middle Albian.
[[..:.'f C o rt e zi ceraij--g.: . ... :'
!YP · ·· .· . S . JitIES. Corteziceras cortezi n.g., n.sp.
- ZvJNctfc.ff.r, -l'1f7t
. · · • · . DIAGNOSIS. Cnch evolute, of moderate
siZe, The early whorls are finely striated the
su.,sequent whorls are divided into segments by
flaring ribs and between them accentuated ribs
occur; the latter may bifurcate at different
levels on the flank. The more prolonged the
initial smooth stage, the less strong the ornament
becomes; all the intermediate ribs are
prominent, however. The ribs tend to gather in
bundles which reach the umbilical rim tangentially
but they do not develop any umbilical
tubercles. There are two constrictions especially
noticeable on the internal mold, parallelling
each flaring rib, of these the adapical constric·
tion is accentuated in early ontogentic stages
but tends to weaken with growth to th·e benefit
of the adoral constriction.
COMMENTS. Corteziceras differs from
Uhligella Jacob 1907 (Type species: D. clansayesense
Kilian) because it lacks nodes or swe·
llings on the umbilical margin. It differs from
Zurchere/la Casey (1954, type species: D. zuer·
cheri Jacob) because of the weakness of the rib·
bing and greater involution of the latter.
.
This genus is named in honor of Santiago
Cortez, Colombian paleontologist·
{r;,fL r:z s cortezj,. n. sp.
lfC3-ng.2, Text-1ig. 4C.
Holotype. Santa Ana, A .. P. G. · · . (U.N.C;M..
N.C.P. Ht. N. 13)
? 1938 Uh/igella ziircheri Jacob &Tobl., Riedel,
p.20, pl. 4, figs. 12, 13, pl. 14, f.4.
? 1957 Uhlige lla ziirchen' Jacob 8. Tobler;
Biirgl, p. 135, pl. 8, fig.S.
The conch is discoidal, moderately umbi."
Heated. The inner whods are smooth, they are
covered only by sigmoidal growth striae; the
latter are concave ado rally on the umbilial wall,
trend obliquely on the adumbilical third of the
flank, then tum feebly adapically describing a
weak arc on the central third of the flank. On
the adventral third of the Dank they trend subradially
and describe an adoral concavity and
finally they barely project onto the venter.
Fascicules of growth lines near the middle
of the flanks pass into ribs that become conspi·
cuous on the venter; at 25 mm diameter they
are already recognizeable. Periodically one of
these ribs becomes more prominent, especially
on venter, but it shows the same trend than the
other ribs; such flange-like ribs are represented
on the internal mold by a rib also, but, immediately
adoral to it there is a deep furrow that
runs parallel to the rib but which is represented
on the exterior by a flattened band.
Adapical to the rib is another less deep
sulcus on the internal mold corresponding to a
flattened band similar to the adoral one. Be·
tween two of these prominent ribs there are
usually 5 finer but also stressed ribs of which
those two occupying a more central position
usually extend down to about the adumbilical
third of the flank, whereas the other two join the
latter at about the central third of the flank;
I