Counting The Number of Whorls
To count exactly the number of whorls in gastropods a straight line is drawn to separate the semi-circular nucleus (protoconch (1 - see image) from the younger part of the shell. An arrow placed at a 90° angle on this line, following the course of the whorl, reaches the end of the first whorl where it is parallel to its starting position. From that point all whorls are counted towards the margin of the shell, estimating the ultimate whorl with an accuracy of a quarter whorl.
Other authors (Ehrmann 1933; Richter & Seapy 1999) applied a slightly different measuring method, resulting in whorl numbers being a quarter higher.
Read more about this topic: Whorl (mollusc)
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