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The periostracum, as well as the coloration, is only rarely preserved in fossil shells. The apertural end of the gastropod shell is the anterior end, nearest to the head of the animal; the apex of the spire is often the posterior end or at least is the dorsal side. Most authors figure the shells with the apex of the spire uppermost.
C. patellitheca is known from broad, low, cap-shaped or mound-shaped shells around 2 centimeters in diameter. The shell has a slight apex that points posteriorly, and is at the posterior end of a central dorsal ridge. [1]
The word "apex" is most often used to mean the tip of the spire of the shell of a gastropod.The apex is the first-formed, and therefore the oldest, part of the shell. [1] To be more precise, the apex would usually be where the tip of the embryonic shell or protoconch is situated, if that is still present in the adult shell (often it is lost or eroded away).
Notched – Nicked or indented, as the anterior canal of some gastropods. [1] Nucleus – The first part or beginning, as the apex in a gastropod shell. [1] Nucleated – Having a nucleus. [1] Obconic – In the form of a reversed cone. [1] Oblique – Slanting, as the aperture of some shells when not parallel to the longitudinal axis. [1]
The aperture is an opening in certain kinds of mollusc shells: it is the main opening of the shell, where the head-foot part of the body of the animal emerges for locomotion, feeding, etc. The term aperture is used for the main opening in gastropod shells, scaphopod shells, and also for Nautilus and ammonite shells.
This is due to the back and forth motion of the shell during movement, which would tend to block the mantle opening against the foot if it was in a posterior position. The evolution of an asymmetrical conispiral shell allowed gastropods to grow larger, but resulted in an unbalanced shell. Torsion allows repositioning of the shell, bringing the ...
A shell of Acus crenulata, with the siphonal notch visible near the bottom left of the image, at the anterior end of the shell. A live individual of the volute species Cymbiola magnifica showing the siphon extended through the siphonal notch of the shell. A siphonal notch is a feature of the shell anatomy in some sea snails, marine gastropod ...
Frequently, the front portions of the shells are broken, though the posterior sections are relatively well preserved. Platyceras is particularly abundant in Devonian deposits (359 million to 416 million years old). [1] Platyceras and other platyceratid gastropods are known for the complex symbiotic relationships they had with crinoids. [2]