Mauritania

• https://www.trufi-association.org/nouakchotts-first-interactive-public-transport-map/: map of public transportation in Nouakchott (not 100 % uptodate). 5 MRU / 0,10 € for each trip. Shared taxi for short distance 10 MRU / 0,20 €. 
• Nouakchott to Nouadhibou: Premiere classe  https://www.1classe.voyage/  is the best option with air condition. 
• cheap journey to Nouadhibou: flights to Mauretania are expensive therefore a flight to Dakhla / West Sahara and bus trip to Nouadhibou (see above) is much cheaper. Budget flights to Dakhla 2 x week with Ryanair from Madrid or with Royal Air Maroc https://www.royalairmaroc.com/de-de with guaranteed fixed prices by the Maroc government including luggage (Cassablanca 100 € one-way two times daily; Marrakesh / Agadir for 80 € one-way 2 x week).
• Nouakchott to Senegal via Rosso: Minibus to Rosso, cross the river with the car ferry (free of charge; if you have to wait too long for the ferry then you can take a pirogue which is fee-paying), then continue with a minibus to St. Louis or Dakar. Bus station is 2 km away from the river.
  
• Nouakchott to Senegal via Diama (close to St. Louis): there are many horror stories in the internet about corruption at the Rosso border crossing (probably it more applies to the tourists who drive with their own car / motorbike) therefore they recommend the Diama border crossing. Perhaps not passable during the rainy season due to the bad road condition.
  
• Nouadhibou to Dakhla / West Sahara: El Mouassavir plus cooperates with CTM Bus from Morocco (departure from Nouakchott or Nouadhibou). CTM Bus daily departs from the border of Morocco at 4 pm. Departure in Dakhla / West-Sahara is daily at 9 am.