The headword is accusative plural, from the quotation given.
[1] Thucydides 3.6.1, here abridged, on the Athenians at Mytilene in 428 BCE. See the note in Charles F. Smith, Commentary on Thucydides Book 3 (the two harbours which Strabo (xiii. 2. 2) describes, were formed, somewhat like the two Syracusan harbours, by an island lying in front of the city. ἐπί, which is wanting with ἐφορμεῖν in vii.3.21, is repeated, for the sake of clearness, with ἐφόρμους ἐποιοῦντο.)