Source : Webster's Revised Unabridged Dictionary (1913)

Pentamethylene \Pen`ta*meth"yl*ene\, n. [Penta- + methylene.]
   (Chem.)
   A hypothetical hydrocarbon, {C5H10}, metameric with the
   amylenes, and the nucleus of a large number of derivatives;
   -- so named because regarded as composed of five methylene
   residues. Cf. {Trimethylene}, and {Tetramethylene}.