different between southwestern vs turtleback

English

south +? western

southwestern (not comparable)

Of or pertaining to the southwest.
Locals think southwestern sunsets are the most beautiful.

Wikispecies

turtle +? back

turtleback (plural turtlebacks)

Anything having the shape of a turtle's back (that is, its shell).
- 1975, Poul Anderson, The Winter of the World
  Up he went on concrete stairs that were eroded to turtlebacks and slippery with evening's frost […]
(nautical) A convex deck at the bow or stern of a vessel, designed to shed seawater quickly.
A primitive stone celt of a form suggesting the back of a turtle.
A library binding of a mass market paperback with a generic hardcover.
(The addition of quotations indicative of this usage is being sought:)
- 1995, The Georgia Librarian (volumes 32-34)
  When you order Turtleback Books from Baker & Taylor, you can be assured you get Turtlebacks!
Any plant of the genus Psathyrotes of annual and perennial forbs and low subshrubs native to dry areas of southwestern North America.

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