Trochoid

sb. and a. [ad. Gr. τροχοειδής round like a wheel, f. τροχός wheel + εἶδος form: see -OID; cf. F. trochoïde (1658 in Hatz.-Darm.).] A. sb.

  1.  Geom. A curve traced by a point on or connected with a rolling circle; orig. = CYCLOID 1: now usually restricted to the curtate and prolate cycloids, traced respectively by points within and without the circle; also extended to curves similarly generated by a circle rolling upon another circle, either inside it (HYPOTROCHOID) or outside it (EPITROCHOID).

1704.  J. Harris, Lex. Techn., I. s.v., A Curve Line … called a Cycloid or Trochoid.

1711.  W. Sutherland, Shipbuild. Assist., 59. The Cycloids or Trochoids.

1867.  Denison, Astron. without Math., 86, note. That curve is called a trochoid, but when the tracing point is on the circumference it becomes a cycloid.

1881.  C. W. Bourne, in Eng. Mech., No. 874. 377/1. The bar AB is jointed to a bar BC, so that while AB revolves round A as centre, BC can also revolve round B as centre, then a curve will be described by the point C…. Every such curve is comprised under the name ‘trochoid.’

  2.  Zool. A gastropod of the family Trochidæ; a top-shell.

1839.  Penny Cycl., XIV., 317/2. Mollusca…. Class III. Gastropoda…. Order 6. Pectinibranchiata. Family of Trochöids.

  3.  Anat. A trochoid articulation, a pivot-joint.

1860.  in Worcester; hence in later Dicts. [Cf. quot. 1857 in B. 3.]

  B.  adj.

  1.  Geom. = TROCHOIDAL 1. rare^–0.

1882.  Ogilvie (Annandale), Trochoid, a. 1. Trochoidal.

  2.  Conch., Top-shaped, conical with flat base, as the shells of the genus Trochus or family Trochidæ; Zool. belonging to the family Trochidæ.

1859.  J. R. Greene, Man. Anim. Kingd., Protozoa, 16. If … the spiral passes obliquely round an axis, the shell assumes a more or less pyramidal form, and is termed ‘trochoid.’

1861.  P. P. Carpenter, in Rep. Smithsonian Instit., 1860, 213. The … African group Collonia have small Trochoid shells.

  3.  Anat. Applied to a pivot-joint, in which one bone turns upon another with a rotary motion.

1857.  Dunglison, Med. Lex., Trochoid..., an articulation, in which one bone turns upon another, like a wheel upon its axle.

1860.  Mayne, Expos. Lex., Trochoides,… resembling a wheel: trochoid. Anat. Applied to a movable connexion of bones in which one bone rotates upon another, as the first cervical vertebra upon the odontoid process of the second.