Step 1: Identifying principal stresses.
In a thin cylindrical shell subjected to internal pressure, the two principal stresses are:
Hoop stress:
\[
\sigma_h = \frac{pd}{2t}
\]
Longitudinal stress:
\[
\sigma_l = \frac{pd}{4t}
\] Step 2: Formula for maximum shear stress.
Maximum shear stress is given by:
\[
\tau_{\max} = \frac{\sigma_h - \sigma_l}{2}
\] Step 3: Substituting values.
\[
\tau_{\max} = \frac{\frac{pd}{2t} - \frac{pd}{4t}}{2}
\]
\[
\tau_{\max} = \frac{pd}{4t}
\] Step 4: Conclusion.
The maximum shear stress in the cylindrical shell is \( \dfrac{pd}{4t} \).