Where is the Nigeria - Cameroon Chimpanzee found?

The West African Chimpanzees are divided from the Central and East African populations by a river called the Sanaga.  The Sanaga is a very unique river - several primate forms diverge on either side of this river (see the article 'Rivers and Wildlife' on Tengwood.org's environment page).  In the case of chimps, the genetics are still being investigated, but it looks as if chimps north of the Sanaga River are genetically distinct from chimps to the south; different enough to classify them as a different subspecies. Another River known as the Niger, along with the Dahomey Gap, may divide the two Western subspecies, and isolate P. t. ellioti from the more Western subspecies, Pan troglodytes verus.  Where the actual boundary lies between these two species is currently unknown - there is not enough information on any chimp populations that may exist West of the Niger River.