The vertebrate retina contains only five major neuronal
classes but these embrace a great diversity of discrete
types, many of them hard to define by classical methods.
Consideration of their spatial distributions (mosaics)
has allowed new types, including large ganglion cells,
to be resolved across a wide range of vertebrates. However,
one category of large ganglion cells has seemed refractory
to mosaic analysis: those that project to the accessory
optic system (AOS) and serve vestibulocerebellar mechanisms
of motion detection and image stabilization. Whenever AOS-projecting
cells have been analyzed by nearest-neighbor methods, their
distribution has appeared almost random. This is puzzling,
because most aspects of visual processing require the visual
scene to be sampled regularly. Here, spatial correlogram
methods are applied to distributions of large ganglion
cells, labeled retrogradely from the AOS in frogs, turtles,
and rats, and to the AOS-projecting displaced ganglion
cells of chickens. These methods reveal hidden spatial
order among AOS-projecting populations, of a form that
can be simulated either by superimposing a single regular
mosaic on a random population or, more interestingly, by
overlapping three or more regular, similar but spatially
independent mosaics. The rabbit is known to have direction-selective
ganglion cells (not, however, AOS projecting) that can
be subdivided into functionally distinct, regular mosaics
by their tracer-coupling patterns even though they are
morphologically homogeneous. The present results imply
that the direction-selective AOS-projecting ganglion cells
of all vertebrates may, likewise, be subdivided into regular,
independent mosaics.