Publicaciones Geolo_gicas Especiales Del lngeominas

ZONATION OF THE CRETACEOUS OF CENTRAL COLOMBIA BY AMMONITES 35
are very prominent but fade out on the adult
earlier than the lateral and adumbilical, B) its
simple, well separated club-shaped ribs, and C)
its subrectangular, dome-like whorl section.
Measurement. D Euw li w DfEuw%D{HO/o!"o. R.
c23·t 25 14 9 1'3· 66 36 ·J0(1/2)
C·223•2 36 16 13 18 44 16
c-22a-3 33 14 15 19 42
43 19
46 19
36
.
C-223-4 12 13 14 40 30
C·223·1i 41 19 16 20 46 19
C·223.0 32 14 12 16 40 38
39
9(1/2)
C·223·7 41 20 16 ? 48 39 19
C·223-8 7 7 7 7 ? 7 9(1/2)
C-223·9 1 1 7 1 ? 7 9(1/2)
43
C·223·10 19 16 21 44 39 22
C·223·11 63 25 20 26 47 38 19
C-223·12 47 21 19 22 44 40 ?
c-223·13 --33 22 20 25 41 38 10(1/2)
C·223·1 4 1 1 ? ? ? ? 10(1/2)
(·223·15 57 26 21 27
44
46 20 37
C:·223·16 73 32 25 36 34 23
COMMENTS. This species is similar to
Cheloniceras (Ep icheloniceros) subnodosocos·
ta tum (Sinzow) [1907, p. 175, text- fig_. 4-5,
pl. 2, figs. 1 • 8, especially plate 2 fig. 8 j, but
on the Russian species is a secondary rib which
arises from the lateral tubercles and a single in·
tercalary also occurs, a charateristic that does
not occur on the Colombian species; the latter
also seems to attain a larger size than Sinzow's
species. From the preceding table it is clear that
the number of ribs is very constant in the Co·
lombian species, contrary to the Russian species
in which this number is variable. The record of
Sinzow's species in the Villa de Leiva by Biirgl
(1954, p. 16) and quoted by myself (1968, p.
28), is now considered doubtful.
OCCURRENCE. Socota Member. Upper
Aptian.
This species is named in honor of Profe·
ssor Carlos E. Acosta, Department of Geocien·
cias, Universidad Nacional. Bogota.
Chelonicera (EpJcheloniceras) jimenqL
n. sp.
Pl. 2, Fig. 8; Text-Fig. 4X.
Holotype. C-514 (U.N.C.M.H.N.C.P.Ht.N.21).
Paratype. C-514 (U.N.C.M.H.N.C.P.Pt.N.22,23)
A) Up to 15 mm D [C-514 ]. The whorl
section is renifonn. The sculpture consists of
veey faint ribs, slmost plaits, prorsiradiate, they
describe a wide arc on the venter; the adapical
half of the last whorl on this specimen has the
shell preserved; the ornament visible on it :s low,
belt-like ribs ventrally arched and separated by
wide intercostal spaces which are covered with
growth striae; fine flat ribs may interpose ado·
rally. The whorl section is a smooth arc. On the
adoral half of the last whorl of this srecimen,
inconspicuous mammiform prominences begin
to form on the ribs at the middle of the flanks.
:rhe ribs begin at the umbilical rim but some
mtercalary ribs begin on the adumbilical third
of the flank. The umbilicus is deep.
B) Up to 21 mm (C· 5141. The ribs have a
tendency to develop a second, faint, radially
elongated tubercle at the periumbilical margin;
on the venter they te··tQ.l\*tpress along the
median line and to raise' sl}gJ:ill!:on both sides;
the ribs are very fine, thread-like to moderately
strong, feebly sigmoidal on the flank and projec·
ted as a wide arc on venter. ..
C) Up to 30 mm D [C-514). The periven·
trai and lateral tubercles accentuate for a short
distance and then soon fade out. Sporadically
some intercalary ribs detach from a node on a
primary at the middle of the flank or from its
periumbilical region. There is a tendency toward
an alternation of a secondary rib between two
primary ones; at the anteriormost observed part
the whorl section is subquadrate.
D) Up to 60 mm D [C-5141. Only half of
a specimen is available on which four whorls
can be observed. The umbilicus is deep, funnel
shaped. On the second internal whorl there are
low ribs which disappear before the umbilical
rim; the umbilical wall is subvertical, high, it
rounds smoothly into the convex sides. On the
third whorl the ribs are rounded imd extend
as sharp ridges to the umbilical" seam, they des·
cribe an adoral ccncavity on the umbilical wall
and part of the adumbilical third of the flank
but on the remainder of the flank they describe
an adoral arc; at this stage of development the
intercostal spaces are two to four times as wide
as the ribs when no intercalary -ribs are develo·
ped.
Intercalary ribs are rare. On the last whorl
the ribs are thread-like, acu te and conspicuous;
they begin near the middle of the umbilical wall
and trend rursiradiate to the umbilical rim where
they bend abruptly, then they trend nearly stra·
ight or in a feeble forward arc on the central
third of the flank, then they cross the venter .
straight or bent forward. On both sides of ven·
ter the ribs raise in a feeble bullae and the me·
median ventral region appears slightly excava·
ted. The intercostal spaces are very wide, cha·
nne!-like, three to five times as wide as the ribs.
The whorl section is nearly rectangular with
rounded periventral margins, flattened venr
and moderately oblique umbilical wall.
DIAGNOSIS. This species is characterized
by: A) the transitory development of the lateral
tubercles, B) the numerous fine, acute ribs on
the adult whorls with nearly no intercalary ribs,
and C) its subquadrate whorl section.
MeiL:Iurements. D Euw H w D/Euw% D/H0k
c. 514 15 6 6 7 40 40
c. 614 21 8 8 10 38 38
c. 514 30 12 12 15 40 40
c. 614 60 30 21 26 50 35
COI\IMENTS. This species is comparable
to "Douvilleiceras Martini d'Orb" as figured by
Sinzow (1906, p.171, pl.2, fig.13-17) but the
latter differs by the stronger development of the