The descriptive word at the root of the term "monotremata" mono (= single) + treme (=hole)

The lowest Order of Mammalia, containing mammals which have only one opening or vent for the genital, urinary, and digestive organs (called the cloaca). The Order comprises the duck-billed platypus (Ornithorhynchus paradoxus) and several species of spiny ant-eaters of the genera Tachyglossus (or Echidna) and Zaglossus or Acanthoglossus. They are native to Australia and New Guinea, and are oviparous (producing eggs that hatch outside the body).

The monotremata are the only known venomous mammals. The platypus has a spur attached to a venom gland on its hind legs.

There are only a few other -treme words in the English language:

  • extreme (unrelated).
  • derotreme (dero = neck) - A type of aquatic amphibians which includes Amphiuma, Menopoma, etc. They have permanent gill openings, but no external gills. Also called Cryptobranchiata.
  • peritreme (peri = around) - A small chitinous ring surrounding a breathing-hole in an insect.
  • postreme (obsolete nonce word) - Last, hindmost.
  • quatreme (unrelated) - A duty or tax of a fourth part levied on certain commodities.
  • streme (obsolete) - Alternate form of "stream".

More information at Dictionary.com.

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