Birotunda

In geometry, a birotunda is any member of a family of dihedral-symmetric polyhedra, formed from two rotunda adjoined through the largest face. They are similar to a bicupola but instead of alternating squares and triangles, it alternates pentagons and triangles around an axis. There are two forms, ortho- and gyro-: an orthobirotunda has one of the two rotundas is placed as the mirror reflection of the other, while in a gyrobirotunda one rotunda is twisted relative to the other.

The pentagonal birotundas can be formed with regular faces, one a Johnson solid, the other a semiregular polyhedron:
 * pentagonal orthobirotunda,
 * pentagonal gyrobirotunda, which is also called an icosidodecahedron.

Other forms can be generated with dihedral symmetry and distorted equilateral pentagons.