A long narrow cut is called a slit.
A long narrow racing boat is called a racing shell.
A long narrow cut could be called a gash or a slash, depending on the context or severity of the wound.
Any "Long narrow arm of the sea" would be called a FJORD.Hope that helps!
A long narrow region is often referred to as a strip or a corridor.
A long narrow part of land that is almost all surrounded by water is called a peninsula.
The ureters are two narrow tubes and each is about 10-12 inches long
A long narrow chain of hills and mountains is called a mountain range or a mountain chain.
The long narrow bone in the lower part of your leg is called the fibula.
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Perhaps a loaf tin.