as are vertices 44 46 with respect to diagonal 45 diagonals 43 45 are parallel to each other corrugations 28 in adjacent rows 30 39 alternately project in opposite directions with respect to the plane of the layer as a result elongated vertices 40 42 are disposed on an opposite side of the plane of the layer from elongated vertices 44 46 elongated vertices 40 42 lie in a single plane as do elongated vertices 44 46 these planes are parallel to each other and to a plane representing the mid plane of layer 24 moreover the planes of vertices 40 42 and 44 46 are parallel to the orientation of rows 30 39 down layer 24 as a result diagonal rows 30 39 of corrugations 28 form alternating diagonal ridges at vertices 40 42 and valleys at vertices 44 46 along each row 30 39 adjacent corrugations 28 are separated by a series of elongated apertures i e slits 48 50 52 54 more specifically vertices 40 42 are spaced by slits 48 50 while slits 52 54 separate vertices 44 46 the vertex e g vertex 40 between two adjacent slits e g slits 48 50 in a row is offset from the adjacent vertices e g vertices 42 each slit 48 54 is elongated in a direction that is orthogonal to diagonals 43 45 and each slit 48 54 terminates before reaching an elongated vertex.