Conus (Conorbis)
dormitor sindiensis (Vredenburg, 1925)
Descrizione e caratteristiche:
Medium-size, biconical, moderately
slender, with rather elongate slightly conoidal spire measuring more than
one-third of the total height. The imperfectly preserved protoconch which is of
very small size, is followed by eight spire-whorls the height of which is equal
to one-third of their width, their maximum thickness being situated relatively
close to their anterior margin. They are separated from one another by thin
incised sutures. The anterior portion of the whorls is convex, while the
posterior part consists largely of a concave surface separated from the
posterior margin by a feebly prominent angular keel. At early stages of growth
the convex and concave portions of the whorls pass gradually into one another,
the convex portion occupying approximately the anterior half of the whorls. On
the later whorls they are separated by a distinct angulation so situated that
the concave portion of the whorls encroaches considerably upon the anterior
convex portion which now becomes reduced to about two-fifths or one-third of
the height of the whorls.
A thin spiral thread corresponds with the
crest of the circumsutural rim and posteriorly bounds the posterior concavity.
The anterior portion of the whorls is decorated with spiral threads slightly
narrower than the intervening spaces, their number being three on the earlier
whorls, increasing to four at later stages of growth in consequence of the
slightly conoidal disposition of the spire, owing to which a slightly increased
surface along the anterior margin of the whorls becomes disclosed with
increasing growth.
Counting from the most posteriorly
situated of this anterior group of spiral threads, it is the second one that
corresponds with the angulation which, at later stages of growth, separates the
concave and convex portions of the surface. The crowded, sharply defined,
fairly evenly distributed, raised lines of growth form a sigmoidal flexure of
which the forwardfacing concavity is situated posteriorly, the point of
inflexion corre- sponding with the angulation which, at later stages of growth,
separates the concave and convex portions of the whorls.
The concave portion of the flexure has a
pronounced curvature, the apex of which is situated at about half the height of
the concave portion of the whorls. Posteriorly it terminates antecurrently to
the suture. The anterior convex portion of the flexure also terminates
antocurrently to the anterior suture. The body-whorl measures slightly more
than seven-tenths of the total height. Posteriorly it exhibits the same
circumsutural rim, and the same concave zone anteriorly bounded by an
angulation as is observed in the later spire-whorls. Anteriorly to the
angulation the shape is, in general, moderately steeply conical, the surface
exhibiting at first a distinct convexity which is best marked close to the
angulation and which gradually atifiens into a straight outline towards the
anterior termination of the shell. The angulation is scarcely further distant
from the anterior than from the posterior termination of the shell which it
consequently divides into an almost symmetrical biconical shape. The anterior
termination is missing, but the anterior portion is nevertheless sufficiently
preserved to indicate that the terminal truncation must have been narrow.
Posteriorly to the angulation the ornamentation, throughout the greater part of
the body-whorl, generally coincides with that of the last spire-whorl though
with the addition of another spiral thread at about half the height of the
concave surface. Anteriorly to the angulation the entire surface is decorated
with flat, ribbon-like, raised threads or bands, narrower than the intervening
spaces and at first all of one width and equally spaced, while, on approaching
the aperture, intercalary threads are apt to appear in many of the intervals.
The lines of growth, posteriorly to the angulation, nre disposed in the same
manner as on the spire-whorls. Anteriorly to the angulation they form a
forward-facing strong convexity ex- tending to the anterior termination of the
shell, and so disposed that it is mostly anteriorly antecurrent, reaching a
vertical trend only when it approaches very close to the anterior termination.
The lines of growth form a delicate crowded web across the floor of the spaces
separating the spiral bands. The aperture, of moderate width, is rather
obscured by a strongly adhering incrustation of hard rock. The outer lip is
antecurront to the suture between which and the angulation it forms a well
rounded sinus. Anteriorly to the angulation it projects forward to a
considerable extent with a strong convexity continuous as tar as the anterior
termination of the shell on nearing which its course becomes vertical and
finally anteriorly retrocurrent,
Dimensions
Height 30 mm.
Thickness 13,5 mm.
Height of spire 11,2 mm.
Height of body-whorl 21,7 mm.
Occurrence. —Nari of Bhagothoro Hill in Sind.
Comparison
Compared with Conorbis dormitor
from the upper Eocene of England, the Sind fossil is perhaps slightly more
elongate than the average of the English shells, with a less conoidal, more
nearly truly conical spire. The difference does not seem sufficiently precise
for the establishment of a distinct species.
Bibliografia Consultata