Petit mal, also known as absence seizures, is a neurological disorder characterized by brief, sudden lapses of consciousness. Given that it is a condition directly related to brain function and neurological activity, the rubric 'Petit Mal' would be found under the chapter dedicated to the Nervous system in Boericke's Repertory of Homoeopathic Materia Medica and Repertory. This chapter encompasses various neurological conditions and symptoms.
The other options are less likely:
\begin{itemize}
\item Mind might contain rubrics related to altered states of consciousness, but a specific neurological condition like petit mal would be more appropriately categorized under the nervous system.
\item Generalities usually includes symptoms affecting the whole body or general pathological states, not specific neurological disorders.
\item Locomotor system deals with conditions affecting movement, bones, joints, and muscles.
\end{itemize}
Therefore, the rubric 'Petit Mal' is located in the Nervous system chapter of Boericke's Repertory.