The nest then takes the form of a demi-ellipsoid, or, in other words, half an egg cut longitudinally; more or less obtuse at one end, but pointed at the other, and sometimes ending in a short curved tail.
In all cases the upper face is convex and regular.

In it we can distinguish three well-marked and longitudinal zones.

The middle zone, which is narrower than the others, is composed of thin plates arranged in couples, and overlapping like the tiles of a roof.

The edges of these plates are free, leaving two parallel series of fissures by which the young can issue when the eggs are hatched.

In a nest recently abandoned this zone is covered with fine cast-off skins which shiver at the least breath, and soon disappear when exposed to the open air.