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\noindent Although radio pulsars appear superficially homogeneous as a class of objects, observational data show two distinct types of radiation source geometries, identified as ``core'' and ``cone.''$^{1} \:$ It is quite striking that the two classes occupy different regions in ($P,\dot{P}$) space, implying that cone emission tends to come from older pulsars, and core emission from younger ones. Furthermore, the two regions are neatly separated by the ``pair death line'' of the Arons \& Scharlemann pulsar magnetosphere model.$^{2}$\\
\noindent This suggests two things to us. First, the cone and core classifications are meaningful, and result from two physically different situations in the polar cap. Second, different emission mechanisms are operating in these two situations. Core pulsar are ones in which a pair cascade is taking place, and coherent radio emission can derive from two-stream-driven turbulence in the dense pair plasma. Purely conal pulsars are a distinct class, lacking a pair cascade. Radio emission must come from a different plasma process taking place on the shear surface at the edge of the cone.\\
\noindent This work was partially supported by NSF grant AST-9315285.\\
\noindent $^{1}$ Rankin, J.M. Astrophys. J. 301 , 901 (1990).\\ $^{2}$ Arons, J., \& Scharlemann, E.T., Astrophys. J. 231 , 854 (1979).
