There are several American genera of _Turdinæ_ not found north of
Mexico as yet, although it is not impossible that one of these
(_Catharus_) may hereafter be detected within the limits of the United
States. The species of _Catharus_ resemble the North American
wood-thrushes (_Hylocichla_); but the spurious or first primary quill
is longer (from one half to one third the second quill), the wings are
rounded, not pointed, the tarsus is longer than the head, and the
outstretched toes extend beyond the tail. The species to be looked for
are _C. melpomene_ and _occidentalis_.[22]
The North American species of _Turdinæ_, while retained under the
single genus _Turdus_, yet constitute several distinct groups, which
we may call subgenera.