42. PIERIDVM LVMEN. At EP III v 29-36 Ovid asked Cotta to send him some of his poetry.
For the sense of lumen here ('ornament'), OLD lumen 11 cites among other passages Cic Sul 5 'haec ornamenta ac lumina rei publicae' and Phil II 54 (of Pompey) 'imperi populi Romani decus ac lumen fuit'.
42. PRAESIDIVMQVE FORI = 'defender of the law'. Compare vi 33-34 'cum tibi suscepta est legis uindicta seuerae, / uerba uelut taetrum singula uirus habent'.
43. MATERNOS COTTAS. This passage should be taken in conjunction with EP III ii 103-8 (to Cotta) 'adde quod est animus semper tibi mitis, et altae / indicium mores nobilitatis habent, / quos Volesus patrii cognoscat nominis auctor, / quos Numa maternus non neget esse suos, / adiectique probent genetiua ad nomina Cottae, / si tu non esses, interitura domus'. The simplest explanation of these two passages is that Cotta had been adopted by a maternal uncle, the last surviving Aurelius Cotta.
The question of Cotta's maternal ancestry is a vexed one; for a full discussion see Syme HO 119-21.
The present passage was written with Prop IV xi 31-32 in mind: 'altera maternos exaequat turba Libones, / et domus est titulis utraque fulta suis'.
44. NOBILITAS INGEMINATA. In a famous study (Kleine Schriften I 1 ff.; trans. The Roman Nobility [1969]), Matthias Gelzer demonstrated that the usual meaning of nobilis was 'descended from a consul'. Cotta was descended from a consul on both sides.
At Met XIII 144-47 Ovid uses nobilitas to mean 'descent from a god': (Ulysses speaking) 'mihi Laertes pater est, Arcesius illi, / Iuppiter huic ... est quoque per matrem Cyllenius addita nobis / altera nobilitas: deus est in utroque parente!'.
44. INGEMINATA. A verbal echo of EP I ii 1-2 (to Fabius Maximus) 'Maxime, qui tanti mensuram nominis imples, / et geminas animi nobilitate genus'.
46. ATQVE INTER TANTOS QVAE LEGERETVR ERAT. This is the end of the sentence that began at 5.