Others were equally sure it was a four-membered ring fused to a five-membered ring, known as a beta lactam.    From Wordnik.com.  [The Nobel Prize in Chemistry 1964 - Perspectives] Reference

OXE has an unstable chemical structure (a four-membered ring containing three carbons and one oxygen) that makes it prone to being split open.    From Wordnik.com.  [Ars Technica] Reference

The double door frame projects pilaster-like, with a four-membered cornice above, from which rises an elliptical arch, with an elliptical cornice about a foot above.    From Wordnik.com.  [The Old Franciscan Missions Of California] Reference

Here it is easy to see the chemical structure, a four-membered ß-lactam ring attached to a six-membered sulphur-containing ring -- derived most probably like the corresponding penicillin, from "� - (a-aminoadipoyl) - cysteinyl valine but in an oxidized form.    From Wordnik.com.  [Nobel Lecture The X-Ray Analysis Of Complicated Molecules] Reference