I set out earlier to find some good examples of barchan dunes, and learned a bunch of stuff... first off, while I think of them as the "iconic" dune, the one that everyone pictures when they think of sand dunes, they're not all that abundant, relatively speaking. Second, they're most often not as perfectly formed as the illustrations in textbooks. Third, they represent sand transport with little net deposition or erosion. That is, while there is clearly erosion and deposition, the sediment input into an area of barchans is roughly equal to the amount that moves out of the area. So the first image is the example I settled on, from near the center of Niger. The second image is zoomed out with approximately the same center as the first; you can clearly see the different dune forms resulting from varying amounts of sand available.

A page at the Great Sand Dunes National Parks website describes the result of larger amounts of sediment as "barchanoid ridges," as the barchans overlap and merge. I think the third picture illustrates this intermediate form; just south of the Arizona border, ~100 km inland from the Gulf of California, and on the northern edge of a large volcanic field that I don't think I've heard about before, this image also shows an inselberg and a relatively young volcanic cone. The last picture is of Great Sand Dunes National Park, which is where I started looking, but was frustrated with what I found; I don;t know what one would call this morphology, but it does look pathological.

Still, for my money, if you want pathological barchans, you need to get your ass to Mars. (Via IAG Planetary Geomorphology Working Group)