After he [Aristaeus] arrived within the overhanging lava-stone roof sheltering [Cyrene's] personal quarters, and she recognized the useless tears of her son. . . .Postquam est in thalami pendentia pumice tecta
perventum et nati fletus cognovit inanes
Cyrene. . . .
est . . . perventum: impersonal use of the perfect passive of pervenio
in . . . tecta: I'm reading tecta as accusative plural, object of preposition of in
pendentia: present active participle, accusative plural, adjectival force, modifies tecta
pumice: ablative singular, some kind of qualification of the phrase pendentia tecta
thalami: genitive singular, a qualification of tecta
As I read it, tecta designates a large structure, the enclosure of, among other rooms, Cyrene's personal quarters (thalami). Aristaeus has arrived in this undersea palace, but in its more public space. Maybe there are other undersea buildings, as suggested by the plural tecta, but this is the one that encloses Cyrene's personal space. My justification placing Aristaeus in the more public space is that the following text concerns libations and a feast, rather that some intimate conference.