Listening to the sound his feet are making ("Crush, crack, crick, crick"), the poet in Stephen recalls a familiar fragment of verse and thinks, "Rhythm begins, you see. I hear. A catalectic tetrameter of iambs marching." Derived from a Greek word meaning "incomplete," a catalectic line of verse is one missing a syllable from the expected structure, in this case iambic tetrameter (x / x / x / x /).
The omission usually occurs at the end of a line, but if "Won't you come to Sandymount" (/ x / x / x /) is scanned as a "tetrameter of iambs" the syllable is missing from the beginning—as if chopped from "O, won't you come to Sandymout." As Stephen continues to brood on the fragment of verse, he chops yet another syllable off the beginning of the second line, to sound out two perfect iambs: "deline the mare."
Stephen's line could be scanned as trochaic tetrameter, in which case the truncated foot would come at the end. In the Keats poem at right, the first and last lines are catalectic trochaic tetrameter, with the missing final syllable marked by a caret. (The second and fifth lines are regular, or "acatalectic," iambic tetrameter, and the middle lines are regular trochaic tetrameter.) "Give me women, wine, and snuff" and "My beloved Trinity" sound metrically indistinguishable from "Won't you come to Sandymount," so perhaps Stephen should be thinking of a catalectic tetrameter of trochees marching. But if Keats' second line were truncated to read "Till I cry out, 'Hold, enough!'," that iambic line would suddenly become catalectic, and sound much the same as the first and sixth. The difference is academic, then—best left to professional prosodists with too much time on their hands.
One of several bizarre, perverse, and indefensible emendations in the 1984 Gabler edition of Ulysses is the alteration of "a catalectic" to "acatalectic." Prosodists might argue about whether the lines Stephen quotes are catalectic iambic or catalectic trochaic verse, but in no rational universe can they be called acatalectic, i.e. metrically regular.