The thumb does not have a middle phalanx (singular for phalanges). There are many similarities between these types of phalanges as far as their shape. The main difference between them is in their size.
Each finger has a sheath covering it and this sheath extends across the palm as well. It can be felt by running your finger over it and is called the flexor tendon sheath. It gives protection to the palm and the surfaces of the finger bones can easily be felt because the flexor tendon sheath attaches at the side of the proximal phalanges as well to the sides of the middle and distal phalanges.
The proximal phalanx has a concave, oval on the proximal surface of the base. This shape allows for articulation with the metacarpal head. Its head has two condyles, separated by a groove. It is slightly angulated toward the palmar surface.
Condyles are the rounded end of a bone. The rounded end of the part of the proximal phalanx forms a joint with another bone called the middle phalanx which has a cup-shaped cavity that allows the movement of the joint.