There isn't one specific instrument called a "shell length measurer." To measure the length of a shell, you would use a standard ruler or measuring tape.

Here's why:

* Accuracy: Rulers and measuring tapes provide precise measurements in millimeters, centimeters, or inches, which are suitable for shell lengths.

* Accessibility: These tools are readily available and easy to use.

* Versatility: You can use them for measuring other objects as well.

Of course, you can also use calipers for more precise measurements if needed.