The Collaborative International Dictionary of English v.0.48:

Ammonite \Am"mon*ite\, n. [L. cornu Ammonis born of Ammon; L.
   Ammon, Gr. ? an appellation of Jupiter, as represented with
   the horns of a ram. It was originally the name of an.
   Egyptian god, Amun.] (Paleon.)
   A fossil cephalopod shell related to the nautilus. There are
   many genera and species, and all are extinct, the typical
   forms having existed only in the Mesozoic age, when they were
   exceedingly numerous. They differ from the nautili in having
   the margins of the septa very much lobed or plaited, and the
   siphuncle dorsal. Also called serpent stone, snake stone,
   and cornu Ammonis.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

ammonoid \ammonoid\ n.
   1. one of the coiled chambered fossil shells of extinct
      mollusks; same as ammonite.

   Syn: ammonite
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

ammonite
    n 1: one of the coiled chambered fossil shells of extinct
         mollusks [syn: ammonite, ammonoid]

Easton's 1897 Bible Dictionary:

Ammonite
   the usual name of the descendants of Ammon, the son of Lot (Gen.
   19:38). From the very beginning (Deut. 2:16-20) of their history
   till they are lost sight of (Judg. 5:2), this tribe is closely
   associated with the Moabites (Judg. 10:11; 2 Chr. 20:1; Zeph.
   2:8). Both of these tribes hired Balaam to curse Israel (Deut.
   23:4). The Ammonites were probably more of a predatory tribe,
   moving from place to place, while the Moabites were more
   settled. They inhabited the country east of the Jordan and north
   of Moab and the Dead Sea, from which they had expelled the
   Zamzummims or Zuzims (Deut. 2:20; Gen. 14:5). They are known as
   the Beni-ammi (Gen. 19:38), Ammi or Ammon being worshipped as
   their chief god. They were of Semitic origin, and closely
   related to the Hebrews in blood and language. They showed no
   kindness to the Israelites when passing through their territory,
   and therefore they were prohibited from "entering the
   congregation of the Lord to the tenth generation" (Deut. 23:3).
   They afterwards became hostile to Israel (Judg. 3:13). Jephthah
   waged war against them, and "took twenty cities with a very
   great slaughter" (Judg. 11:33). They were again signally
   defeated by Saul (1 Sam. 11:11). David also defeated them and
   their allies the Syrians (2 Sam. 10:6-14), and took their chief
   city, Rabbah, with much spoil (2 Sam. 10:14; 12:26-31). The
   subsequent events of their history are noted in 2 Chr. 20:25;
   26:8; Jer. 49:1; Ezek. 25:3, 6. One of Solomon's wives was
   Naamah, an Ammonite. She was the mother of Rehoboam (1 Kings
   14:31; 2 Chr. 12:13).

     The prophets predicted fearful judgments against the Ammonites
   because of their hostility to Israel (Zeph. 2:8; Jer. 49:1-6;
   Ezek. 25:1-5, 10; Amos 1:13-15).

     The national idol worshipped by this people was Molech or
   Milcom, at whose altar they offered human sacrifices (1 Kings
   11:5, 7). The high places built for this idol by Solomon, at the
   instigation of his Ammonitish wives, were not destroyed till the
   time of Josiah (2 Kings 23:13).