On two types of pulsars.

Several arguments support the classification of pulsars as either (1) short-period, described by the Smith model with the radio emission generated near the light cylinder, or (2) long-period, conforming to the hollow-cone model, the radio waves being emitted fairly close to the neutron star surface. The pulse width and the overall change in the polarization angle along the average pulse profile depend differently on period P for the two pulsar types; (b) only pulsars of the first type show an interpulse; (c) the rms deviations in the pulse arrival times and the radio beam patterns differ; (d) the two types may have distinct spindown mechanisms. Data for the new PSR 1937 + 21 (P = 1.56 msec), a typical representative of the first group, are compatible with this concept. A possible scheme of pulsar origin and evolution is outlined.